Unfortunately I only remember a few small details about this movie. It's a children's cartoon movie and seems like it was from the 80s possibly early 70s but seemed like the same time as movies like The Black Cauldron, The Princess and the Goblin, Little Nemo. All I remember is that there was a kingdom of sorts and an evil character locked away by a king
Maybe? The evil character was all black kind of looked like ink and was misshapen. It was a lot of trouble to lock this evil character away and there was a key that either a boy or girl I can't remember was never supposed to unlock the door where this evil character was trapped. But that boy or girl did
And let it loose over the whole kingdom. The movie was
Kind of dark for a kids movie is what I remember. Hopefully
Someone remembers this movie!

You said the name of the movie in your question. The movie is little nemo. I actually still have this movie somewhere on vhs. It was one of my favorites as a kid. I believe there are clips from it on YouTube.

Actually, there's more to the title.

Little Nemo: Adventures in Slumberland (1989)
IMDb synopsis:
A young boy whose dreams transcend reality is sucked into his his own fantasy, which is everything he has dreamed of until he unleashes a century-old secret that may destroy not only this perfect dream world but reality itself.
From Wikipedia:
King "Morphy" Morpheus is the king of Slumberland and the Princess's father. He ordered the Professor to bring Nemo to Slumberland to be his daughter's playmate. He shares an interest of trains and machinery with Nemo, who grows very dear to him.. . Morpheus decrees that Nemo is to be trained to be Prince of Slumberland and gives him "The Golden Key", which can gain him access to anywhere in Slumberland, provided Nemo promises not to open the door with the same symbol as the key on it. He gets kidnapped by the Nightmare King at Nemo's coronation, prompting Nemo to set off after him, along with the Princess, Professor Genius, Icarus, and Flip.

Little Nemo: Adventures in Slumberland (1989) - IMDb (http://www.imdb.com/title/tt0104740/)
Little Nemo: Adventures in Slumberland - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Little_Nemo:_Adventures_in_Slumberland)

Here's the movie on YouTube:
Little Nemo Adventures In Slumberland (1989 Full Movie) HD 1080p - YouTube (http://www.youtube.com/watch?v=pGAt0GQ703U)