The Eternal Undead Source/Escape the Creepy Citadel

Here is how the search for the Eternal Undead Source begins in Rise of the Portal Masters.

Soon, Twilight, SpongeBob, Mickey, Thomas, Mumfie and their friends placed the Skull Mask on the Core of Light

Twilight Sparkle: It's done.

Master Eon: Excellent. The Skull Mask of the Philosopher King Mortal Arts, First Lord of the Undead.

Henry: (scared) I hope nothing evil will happen once we find the Eternal Undead Source.

Gordon: Don't be ridicules, Henry. There's nothing to worry about, Some Undead can be a good thing.

James: