Complex sentence: Skeleton King is an evil undead warlord/sorcerer is the main antagonist of the series who resides in the Citadel of Bone, a gigantic living spaceship made entirely out of bone filled with nightmarish creatures.
Split into simpler sentences: Skeleton King is an evil undead warlord/sorcerer. And is the main antagonist of the series who resides in the Citadel of Bone, a gigantic living spaceship made entirely out of bone filled with nightmarish creatures.