It was announced last year, as part of a whole slew of announcements of live action/cgi remakes of animated Disney classics
that Disney is going to make The Chronicles of Prydain into a film series.
https://variety.com/2016/film/news/chronicles-of-prydain-movie-disney-1201733058/#! 

The Chronicles of Prydain is a series of books written by Lloyd Alexander



In 1971 Disney purchsed the rights to the books, and started delveolpment on the story and designs.
They had high hopes for it, they wanted it to be the next Snow White.
Mel Shaw did some fantastic concept art which can be be found in Disney related books, Blogs websites and in the bonus features section.
It had been through devleopment hell until 1980.

more is told on cartoonbrew (I'm up late at night here as I'm typing):
http://www.cartoonbrew.com/untold-tales/mouse-in-transition-cauldron-of-confusion-chapter-10-102743.html 

excerpt from The The Illusion of Life 1981



Note: Haley Mills was to voice Princess Eilonwy.
But in the final film the princess was voiced by Susan Sheridan (Noddy)




backstage at disney 1983

note in this documentary according to John Culhane that The Black Cauldron was planed to be released in 1985. and it did. 
another note Producer Joe Hale metions Jonathan Winters as King Eidilleg. (Jonathan was replaced by Arthur Mallet) (Secret of Nimh)





Interview with two voice actors for the film John Bryner (The Ant and The Arddvark) and John Hurt (The Lord of The Rings 1978)


Here on CD on the amazon website is Elmer Berenstein's Music Score for The Black Cauldron.
https://www.amazon.com/Black-Cauldron-Elmer-Bernstein/dp/B007SYIQAM/ref=pd_ybh_a_36?_encoding=UTF8&psc=1&refRID=32PKAF2DMV4D4GQPMAP4 




The film didn't do so well in theaters, and as a result it became one of those hot potatoes that Disney would not make available on home video. for some years.

Here is something of interest in the promotional materials the list of the Disney Animated Feature Canon is included
http://auction.howardlowery.com/Bidding.taf?_function=detail&Auction_uid1=3777460 


Finally The Black Cauldron got a home video release on 1997 in the UK and in 1998 in the US due to fans wanting to see the film

One of Walt Disney's top animators Milt Kahl came out of retirement and did some designs for the film . only Taran and Eilonwy were used
http://andreasdeja.blogspot.com/search?q=milt+kahl%27s+black+cauldron 

July 24th, the 32 anniversary of the release date of The Black Cauldron

Deleted alternate Fair Folk scene


interview with producer Joe Hale
http://animatedviews.com/2010/the-black-cauldron-producer-joe-hale-talks-munchings-and-crunchings/ 

Cauldron of Chaos articles by Micheael Peraza
http://michaelperaza.blogspot.com/search/label/Black%20Cauldron 

Den of Geeks article on Disney's dark phase in both live action and animation
http://www.denofgeek.com/us/movies/disney/260524/exploring-disneys-fascinating-dark-phase-of-the-70s-and-80s 

interview with Glenn Keane
May 2, 1997 by Didier Ghez
http://www.aimeemajor.com/anim/dkeane.html 

I read someplace that Jeffrey Katzenberg actually went into the cutting room and edited the parts he wanted out himself. Sounds pretty outrageous, but is it true? (And if so, shouldn't he get a film-editing credit here?).

Heritage Auctions has The Black Cauldron model sheets and photostats of storyboards
https://comics.ha.com/itm/animation-art/model-sheet/the-black-cauldron-model-sheets-and-storyboard-print-group-walt-disney-1985-/a/7171-97134.s?ic3=ViewItem-Inventory-BuyNowFromOwner-ThisAuction-120115 

Floyd Norman: Being back in the Disney family meant I could attend screenings of the ill fated animated feature and each new screening grew successively worse.
The directors began shifting the order of sequences as if that would garner a more compelling narrative.
Sadly, nothing appeared to help and the arrival of new Disney management in 1984 only drove the nail deeper.

http://floydnormancom.squarespace.com/blog/2015/4/1/the-black-cauldron 


Jeffrey Katzenberg: In the first couple of weeks I was at the studio,
I saw The Black Cauldron. It was a very dark and a very troubled movie.
“This is just way, way, way too violent and too scary. You have to edit some of these things out”
They said, “Well, you can’t edit an animated movie.” I said, “well, of course you can.” And they said, “No you can’t.”

From the documentaries Waking Sleeping Beauty (2009) and The Making Of The Little Mermaid (2006)


Michael PerazaI: I recently spoke with Producer Joe Hale and asked him for his recollections about "Cauldron."
Joe's long Disney experience included being Ollie Johnston's assistant on "Peter Pan" working on Smee and later with Woolie while on "Lady and the Tramp," 
 and Ward Kimball on "Toot, Whistle, Plunk and Boom." It was on "Sleeping Beauty" that Joe moved over into the Layout department and under Don Griffith's mentoring.
Joe had originally been doing story development on Cauldron working closely with Vance Gerry and Mel Shaw.  When asked by Ron Miller to take on the role of Producer,
he turned it down not wanting to step on toes or have to deal with the mounting politics in the studio. 
Of course he did eventually take on that mantle when frustration rose within the crew and someone had to step up to the responsibility.  
Later after the new management team came on board, he faced yet another level of frustration. "When Katzenberg first screened the film (Cauldron) he told us to cut it by 10 minutes.  
Roy Disney and I got together and found some scenes we could get rid of that didn't affect the story that much."
When they ran it agin for Jeffrey and the film finished he asked Roy, "Is that 10 minutes?"
When Roy replied that no it was only around 6 minutes.  Jeffrey stated, "I said 10 minutes!" 

Joe continued, "Eventually he (Jeffrey) cut out about 12 minutes which really hurt the picture. " 
I'll jump back in and add that It's always an expensive and intensely muddled action when editing an animated feature after it is in full color.
Those steps were always meant to be edited while in the storyboard stage or at least before animation.
I'd rather see a story or layout guy do a hand full of drawings and test the flow on a leica reel than an animator slave over a hundred pages of sweat only to see it cut out of the picture.
Going all the way into final color and then making those decisions is just ludicrous. Of course even the classic films have their "soup eating sequences" so it is not unheard of to edit after animation,
just an unfortunate screwup when it does occur. Joe received an early copy of the new DVD release of "Cauldron" yesterday and he and his lovely wife Bev informed me that the image is sharp, bright and colorful.  
They also briefed me that they included about 8 minutes or so of previously unseen footage, mainly of the Faire Folk sequence that was cut before the film was released.  

http://michaelperaza.blogspot.com/search/label/Black%20Cauldron 

Would you like to check out my Prydain blog
https://art-ofprydain.tumblr.com/ 

Bing Translate of of a Norwegian-Bokmal interview with Ollie Johnston:
I think we got back on the right track with Bernard and Bianca-it had a lot more heart than Aristokattene and Robin Hood. So, on two good friends (1981) we worked only in one year,
the most on the screenplay section and to establish gåsykluser and such for the characters. They did not have enough people on the production and I don't think the script was as good as it should.
Then they destroyed the Taran and the black cauldron (1985). Indeed, it was Frank and I, who persuaded the Studio to bet for the project, but the leadership at the time didn't appreciate what the film could offer,
and everything went from each other. It was horrible for it could have been just as good as snow white.


http://www.norskanimasjon.no/pub/index.php/ffaf/artikler/ollie_johnston_et_intervju_del_1 

I like some help with my attempt at debunking misinformation:

Wikipedia: "Informed at what Katzenberg was doing by Hale, Disney CEO Michael Eisner called Katzenberg in the editing room and convinced him to stop.
Though he did what Eisner insisted, Katzenberg requested that the film be modified,
and delayed its scheduled Christmas 1984 release to July 1985 so that the film could be reworked."

https://en.wikipedia.org/wiki/The_Black_Cauldron_ (film)

hmmmmmm

Excerpt from Disney Film Behind Schedule August 10 1978
by Aljean Harmetz

Los Angeles - "The Black Cauldron" Walt Disney"s $10 million animated film scheduled for 1980, is four years behind schedule.
It will not be completed until Christmas 1984,
because the new crop of young animators the studio has spent six years acquiring are not yet competent to handle in complexities.
"The Black Cauldron" which is based on Lloyd Alexander's interpretations of medieval Welsh mythology,
will be replaced in 1980 by a simpler and easier movie about animal freindship, "The Fox and the Hound."

https://news.google.com/newspapers?nid=1310&dat=19780810&id=9AZFAAAAIBAJ&sjid=6uEDAAAAIBAJ&pg=7122,2605529&hl=en 

Lets recap:

"It will not be completed until Christmas 1984,"
If you watched the classic Disney Channel documentary Backstage at Disney (1983) hosted by John Culhane,
John mentions that The Black Cauldron was set to be released in the summer of 1985.

So if completion date of Christmas 1984 was the delayed release date, Did they change the release date long before Eisner, Wells, Disney and Katzenberg arrived?
was the summer 1985 date planned later after the 70's ?

"Informed at what Katzenberg was doing by Hale, Disney CEO Michael Eisner called Katzenberg in the editing room and convinced him to stop.
Though he did what Eisner insisted, Katzenberg requested that the film be modified," Did Eisner do that?
If anyone in this fourm site that worked on the film witnesed such a thing, please prove me wrong.

Sources of info for the wiki entry:
http://www.slate.com/articles/arts/dvdextras/2010/10/the_black_cauldron.single.html 

and James Stewart's book Disney War (which the Slate article also uses! DOH!)

From Sarah's Black Cauldron Page
http://www.reocities.com/tenar_99/index.html 

Writing the Music

Bernstein considers his score for The Black Cauldron one of his most important and challenging assignments in his 35 years as a motion picture composer.
This score marks Bernstein's first grand symphonic score in nearly a decade. Before The Black Cauldron Bernstein wrote music for Ghostbusters, National Lampoon's Animal House, Airplane, Stripes, and Trading Places.
"One thing that was very attractive about The Black Cauldron, outside the fact that I love the film, was that this project lends itself to an ambitious score," he said.
"Many motion pictures have similar requirements; westerns and comedies have their own set of considerations and problems. In any career, The Black Cauldron would be an unique endeavor."
When Bernstein first saw The Black Cauldron, it was still in rough form with pencil test footage. Bernstein as still able to get the full impact of the story.
"I started working on the themes for the main characters, mostly Taran and the Horned King. Taran is an Assistant Pig-Keeper who wants to be a hero. Hie theme at the very beginning is sweet and light.
You would never guess at that point that that theme could become heroic but it begins to sound very different when he has the magic sword in hands. This was donw through orchestration," recalled Bernstein.
One of Bernstein's special indredients for adding magic to the score was the use of a rare French instrument called the Ondes Martenot.
Since no one in the United States and very few people in the world for that matter could play this instrument, and English musician by the name of Cynthia Millar was flown over for the recording session.
Millar had previously played the Ondes Martenot for the score of A Passage to India.
"This instrument was invented in France in 1928 and was one of the first electric instruments," said Bernstein.
"It sounds like maybe everything and nothing exactly. There are times when it might sound like a flute, other times when it might sound like a human voice or a cello.
I used it a lot in connection with the main characters Taran and Eilonwy."
Bernstein describes the Horned King's theme as overwhelming and very menacing. The witches were another matter entirely. To highlight their eccentric and comic nature,
Bernstein created "weird off-center waltzes with some suggestion of rock and roll."

In an attempt to find an appropriate use for the obviously talented Burton, Disney decided to make him a concept artist and team him with another young animator, Andreas Deja.
“They were very nice to me,” Burton said. “They said, ‘We’re doing this movie, The Black Cauldron’, so I just sat in a room for a year and came up with ideas and stuff,
just drew any idea I wanted to, and it was great. It was like weird characters, weird props, weird furniture, just sitting in a room doing whatever I wanted.
But at some point I realized they had no intention of using any of it. It was like that TV show, The Prisoner. It was all very pleasant, all very nice,
everyone’s smiling and being very supportive. But it’s like you realize early on that it’s like a vacuum, a black hole. When I was at Disney, animation was in a terrible state.
I just wanted to get out. The talent was there, but they didn’t have the foresight to see that people have a sense of quality and would respond to it.”
It was really not a collaboration between Burton and Deja. Their styles and personalities were very different. While some of Deja’s designs made it into the final film,
none of Burton’s work did. All of that work is owned by the Disney Studio and if they were clever they could release a book of Burton’s imaginative sketches for the film,
including a Burton creature that is created by four distinctive animals when it is frightened.

https://www.mouseplanet.com/9941/Tim_Burton_The_Early_Disney_Years 


Jim Korkis’ Animation Anecdotes #151:
Apparently, the animators and producer Joe Hale wanted to use young Tim Burton’s distinctive designs for “The Black Cauldron” (1985)
but directors Art Stevens, Ted Berman and Richard Rich felt those drawings were not “Disney” and kept pulling at Hale to forget about them. Finally,
the issue was taken to President and CEO Ron Miller to decide since the animators wanted to make the film quirky but the directors wanted a more traditional film.
Miller had just seen the outstanding box office results for the re-release of “Lady and the Tramp” and decided that Disney should not deviate from the traditional path.

http://cartoonresearch.com/index.php/animation-anecdotes-151/ 

Didier Ghez's interview with Glene Keane

DG: Can you tell me about this work on The Black Cauldron that you did?
GK: John Musker was working on a sequence which was in the witches house. He was designing it so that all the backgrounds were optical illusions like M.C. Escher.
Now, Tim was doing these character designs, a lot like Nightmare Before Christmas type characters. At what point he designed the gwythaints. The gwythaints were like pterodactyls, which is what they look like in the film now, pterodactyls. But Tim designed them so that their heads were really hands and he put their eye right there between the thumb and the forefinger. So they looked like, you know, when you make little silhouette figures on the wall. These things would come flying at you, but then could also grab and they had a snake-like tail and wings. Wild, great ideas!
The Horned King was more of a psychotic, schizoid guy. You heard his two different personalities by puppets. He would have these two different puppets and he was a ventriloquist. One puppet would say... like if he was considering killing somebody, one puppet that was like a psycho clown would say (loud, crazy voice): "Get him! Get him! Yes! Yes! Get his head off! Get his head off!" and the other head puppet would go (soft, squeaking voice): "No! No! let him live! let him live!". The Horned King was just a completely twisted, bizarre character. Now, he is just what we call the Evil Bonehead (laughter).
DG: What were the scenes that you worked on?
GK: I was doing some animation of fairies. I designed a lot of different characters on the thing. I did some animation of Gurgi, Eilonwy and did some experimental animation on them. This is a scene of Eilonwy, where she is picking things out of Gurgi's hair. She is talking.
I loved the voice of this character. I came upon a whole different kind of design on her, but the director did not want something that was so cartoony. Everything I did was being thrown out. They just did not like anything I was doing. Eventually the directors asked me if I would just leave the film and go do something different.
So I did Mickey's Xmas Carol. I worked on the Giant. Ron [Clements] And John [Musker] were also being kicked out of the film and they went to work on The Great Mouse Detective.

http://www.aimeemajor.com/anim/dkeane.html 

Ron: “We were both sort of united on Caldron,
from the standpoint that we had certain ideas for the movie that were not embraced, and we kind of became sort of…”

John: “We were the odd men out,
along with a few other people who wanted the story and the characters to go in a certain way,
and the people in charge didn’t see it that way. It was very frustrating.”

Ron: “We were exiled. We were basically banished from the movie.”

John: “They said, ‘You guys have a different idea, go off and do it.’”

Ron: “So we were both exiled from Cauldron and we both worked together on Great Mouse Detective. Ron Miller, who was the head of the studio at that point,
was the producer of Great Mouse Detective.
We had been working on the story for maybe a year and a half,
and Ron just disappeared.”

John: “He was busy fending off a takeover.”

Ron: “He never showed up, and we became this little floating island, [wondering], ‘Does anybody know that we’re here?’
Finally, Michael Eisner came, and Jeffrey Katzenberg, and Roy Disney came back, and we had to pitch our Great Mouse idea to them as if it were a brand new project,
even though we’d been working on it for a year and a half. They liked it and wanted to do it.”

http://www.animationmagazine.net/top-stories/john-and-ron-mention-the-unmentionable/ 

A question for those who have been in the film industry: What is the difference between a movie completion date and a movie release date?

I know that MGM's "Phantom Tollbooth" was completed in 1968, but not released until 1970.

On a live-action note, I know that Eddie Murphy's "A Thousand Words" spent four years on the shelf before it came out in 2012.

Do the animator drafts still exist for this film?

excerpt of an interview with animator Kathy Zielinski by the animation guild
transcribed by Tumblr user: hellyeahprydain
"
A Conversation with Kathy Zielinski
She talks about her work on “The Black Cauldron” from 13:50 to 16:05, if you prefer a transcript:
Kathy Zielinski [KZ]: […] And then, from there, went on to “The Black Cauldron” which already, I guess, some animators had been working on for several years,
I think even before I had started there.
Steve Kaplan [SK]: So, what did you animate on “The Black Cauldron”?
KZ: Black Cauldron, I did the three witches.
SK: Oh ok.
KZ: Yeah, yeah, and my—
SK: So, they were fun characters.
KZ: [laughs]
SK: In the picture?
KZ: Yeah, yeah and I’ve got some very embarrassing scenes that I did in that film. It was the frog that got stuck on the witch’s cleavage.
SK: Oh…
KZ: It was bouncing around.
SK: They gave that to you?
KZ: Yeah, they gave that to me. Lucky me! I mean, I did a lot of things on the witches but that was… The one.
SK: One of the few that ended on the picture.
KZ: [laughs] Yeah, it was really fun. It was even more fun getting issued the scene by Ted Berman.
SK: Did he? Act embarrassed?
KZ: Yeah, he had to… Well, he didn’t act embarrassed, well,
he was embarrassed certainly because he was having to describe what the scene was and it was really funny because he was saying
“Ok Kathy now you got to animate this frog moving around in the jugs”
[both laugh]
KZ: In the jugs! I was like, “Isn’t that going to—”
SK: Bottles?
KZ: “And huh, and her breasts are called jugs? Ok.” [laughs] But, that was hilarious, so yeah. So, those were,
you know… I actually haven’t seen Black Cauldron since it came out, I should watch it again but I probably would have a pretty good laugh [laughs]
SK: Yeah, it’s just two years I’ll never get back.
KZ: Yeah, yeah, and that picture, god, that went on for a long time and I remember it was at a time where you could spend forever doing a 2 foot scene,
I mean there was no footage quotas. At all!
SK: Oh yeah.
KZ: I don’t recall.
SK: Well, it was kind of the end of the old era.
KZ: Yeah.
SK: Where Frank and Ollie were doing 60/80 feet a week and all the, well, you know,
the relatively green people were doing 3 or 4 feet a week and some less than that and its… Yeah it was an interesting time.
[and then they go to talk about Kathy’s training with Eric Larson]
Part 2 (if anyone’s interested in listening to the full thing)"

https://animationguild.org/oral_history/kathy-zielinski/ 

http://hellyeahprydain.tumblr.com/post/14718613459 

"Jim Korkis Animation Anecdotes #128:
Back in 1984, singer Michael Jackson persuaded the studio top brass to give him a private advance screening of the then-unfinished animated feature, “The Black Cauldron”.
Michael cheered all the way through the film.
(And there is a good story in here somewhere about how Disney tried to remarket the film as “The Dark Cauldron” and then later “Taran and the Magic Cauldron”.)"

http://cartoonresearch.com/index.php/animation-anecdotes-128/ 

Jim  the only evidences I can  find is for Taran and The Magic Cauldron.
The alternate title can be found in children’s books, puzzles, a poster for a limited rerelease from the early 1990′s
and a English title card in the French dub on the Gold Classic Collection 2000 dvd.

on Disney animator Andreas Deja's blog, he mentions Marc Davis also contributed designs for the film.

http://andreasdeja.blogspot.com/2017/12/more-early-black-cauldron-ii.html 

Jim Korkis' article on the behind the scenes of Mickey's Christmas Carol
https://www.mouseplanet.com/11965/Mickeys_Christmas_Carol 


some mentions of storyboard artist Burny Mattinson: having been assigned working on Fox and the Hound (1981)
and early development on The Black Cauldron (1985)  

Jim Korkis:  After graduating high school, Burny Mattinson took a job in “Traffic” (generally delivering mail and doing errands) at Disney Studios in 1953 hoping to someday pursue a career as an animator.
He became an inbetweener for the animated feature Lady and the Tramp (1955). He worked his way up to assistant animator working under Disney Legend Eric Larson. He became a character animator on Robin Hood (1973)
.However, Mattinson’s real passion and expertise was storyboarding. Disney Legend Frank Thomas saw Mattinson’s thumbnail sketches for Winnie the Pooh and Tigger, Too (1974) and asked him to help on the storyboarding for the animated feature The Rescuers (1977).
That opportunity led to him storyboarding The Fox and the Hound (1981) and The Black Cauldron (1985).He and Mel Shaw had done a lot of storyboard work on The Black Cauldron, compiling all five books of the Prydain Chronicles, but during the production the story started to veer wildly from those boards.
Burny’s wife, Sylvia (who was also a Disney animator), saw how unhappy he was so encouraged him to pitch his own project. Mattinson had always wanted to do a film with the Fab Five (Mickey, Minnie, Donald, Goofy, Pluto) and was familiar with the Christmas Carol record album. He sent it to CEO Ron Miller, along with his suggestions. Two days later, he was surprised when he was called into a meeting with Miller who started the encounter by grumbling about why Mattinson had sent him this material.
Mattinson felt he had overstepped his bounds and might lose his job. Miller was just joking. He thought it was a great idea and gave Mattinson the approval to produce and direct the project.It was Mattinson’s first directorial assignment, but was so successful he later went on to co-direct The Great Mouse Detective (1986).

from DIsney animator Andreas Deja's blog, A colection of different character designs of gurgi by Tim Burton, Milt Kahl and Hendel Butoy

http://andreasdeja.blogspot.com/2017/12/gurgi.html 

the animation guild's interview with Disney animator David Block


David Block: I Came off of Fox and the Hound, and there were two movies getting ready to go. One was The Black Cauldron, which everyone was so excited about,

Steve Hulett: yeah, yeah

David Block: and the other one was Mickey's Christmas Carol

So in those days if you wanted to lobby for yourself, you did a personal test on your own time.

working nights and weekends just you wern't established yet you wern't the ron clements or john polmeroys or that ilk

You had a sort of win them over that you could do something.

Steve Hulett: sure

David Block: so i did a test of the hero from black cauldron and I went in to show the director, one of the directors who remain nameless

and he looked at that and said "Oh thats thats much broader then anything we're going to be doing, we're going to go very realistic."

Steve Hulett: it was the sleeping beauty snydrum

David Block: So I came out of there and went" I gotta get off this movie, I gotta get on Mickey's Christmas Carol"

so I did get on Mickey's Christmas Carol. and that was phonominial. aah god what a blast. and I was among really great animators. Glenn was on it. ed gombert, randy cartwright, mark henn, dale bear

I was in a very select group of animators, and we had a great time, the picture came out great. and became kind of an anual event when the thing would show up on tv

and more importantly kept us off of black cauldron for two years


Steve Hulett: yeah

David Block: That was fa fantasitic because that thing was going into the toilet real quick

Steve Hulett: yeah

David Block: so mark and I finished Blac uhh Mickey's Christmas Carol together and went on to black cauldron together and we even went on to the same sequence together

Steve Hulett: yeah,

David Block: and...

Steve Hulett you end up you still got it

David Block: we still got it but we gotta see some characters by Black Cauldron standards. we ende up doing this character Fflewder Fflam

Steve Hulett: he was good

David Block: he was a comic relief

Steve Hulett: creeper was too that were comic relief

David Block: right and Gurgi was also comic relief. but anyway we got htis character.
so we got this character we worked on him and we got off early to work on basil of baker street.

Steve Hulett: yeah

David Block: which became great mouse detective we got on to that early


https://animationguild.org/oral_history/david-block/