8

I remember seeing the movie when I was like 8 or 9, 2008/09, but it could be older than that.

It was about this kid who found himself in another world. I forget if he was taken, teleported, or what. But the King holds a huge parade in his honor. He meets the princess; they're supposed to marry I think. And the kid's shown a really big door he's not supposed to open, and is told very clearly DO NOT open it, and then given the key I think, that or he takes it later. After the parade when everyone is sleeping he goes and opens the door because he's curious about what is inside. And the great big bad comes flooding out and makes a mess of everything. I want to say he was this shadowy embodiment of evil or something, but I can't really remember anything besides the parade, door, princess, king, and that it starred the boy.

10

This is likely Little Nemo: Adventures in Slumberland. Relevant plot excerpt:

Nemo is taken to Slumberland in a dirigible which he is allowed to drive, causing some chaos and is introduced to King Morpheus, who doubles as the circus ringmaster on Earth. Morpheus reveals that he summoned Nemo to become his heir to the throne. Morpheus gives Nemo a golden key that opens every door in the kingdom and warns him of a door with a dragon insignia that must never be opened. Nemo is introduced to Princess Camille and the pair roam the entirety of Slumberland together. Afterward, Nemo meets the mischievous clown, Flip, who angers a group of cops and forces him and Nemo to hide out in a cave. There, Nemo discovers the door that Morpheus warned him not to open. Flip tempts Nemo into unlocking the door, which unleashes the dreaded Nightmare King. Nemo rushes back to Morpheus' castle in time for his coronation ceremony, where Nemo is handed the royal scepter, the only thing capable of defeating the Nightmare King should he ever return to Slumberland. In the middle of a dance session between Morpheus and Genius, the Nightmare King reaches the castle and steals Morpheus away. As the partygoers search for a scapegoat, Flip reveals that Nemo was responsible for the Nightmare King's escape, since Morpheus gave him the key.

The Trailer

Here is where the Nightmare King escapes:

Found by searching for animated film boy opens door and lets out evil

  • 2
    Aw man... completely forgot to add that you can accept a correct answer by clicking on the checkmark by the voting buttons. – FuzzyBoots Jun 17 '18 at 3:29

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