I remember watching this movie when young, all I can remember is that a king gives a boy a key to a special door in the dungeon and tells the kid never to open it. Well it goes on to be that the kid (with the help of an unsual character, maybe a troll) opens the door and an evil inky blob overtakes the castle. What is this movie?

It's "Little Nemo: Adventures in Slumberland" (1989)

I swear this is the number 1 asked about movie.

Thank you so much! That has been bugging me for years! I'm definitely putting this site on my favorites.