Villains whose appearances are teased/mostly left to imagination until the end when they're properly shown

Oogie Boogie - All you see of him is his shadows until the moment he starts singing to Sandy Claws about the torment he's about to face

Firelord Ozai - You mostly see ominous silhouettes of him, a few times maybe even a tapestry of him, but it takes a lot of suspense (for both the audience and Aang) to actually meet him for the first time to face against him