The Giant Goo Skull is a giant monster that appeared in "Vault of Bones." In "Vault of Bones," Flame Princess started wreaking havoc in the dungeon full of bones. In doing so, she angered a legion of Goo Skulls which attacked her. She easily defeated them, since their goo is flammable. However, the Giant Goo Skull appeared and captured Finn, taking him away. After Flame Princess noticed this, she went after the giant.
The monster started wrapping Finn in its goo. Flame Princess ordered him to let Finn go, but the Goo Skull didn't listen, angering Flame Princess. However, instead of killing the monster, she made him do the splits (like Finn previously did against a skeleton). Then, Flame Princess freed Finn from the goo he was wrapped in.
The Giant Goo Skull looks exactly like a regular Goo Skull, except much larger. He also has chainsaws tied on each part of his body.
- The Giant Goo Skull may be the leader of the Goo Skulls.