The Vault of Bones is a dungeon found in a forest near the Tree Fort. As the name suggests, it is filled with bones and is inhabited by sentient, possibly undead Skeletons (one notable skeleton has a big head and lost his arm in a pool of water), a hologram that looks like a frightening monster with a giant sword and shield, legions of Goo Skulls, as well as an unnamed ostrich-like skeleton that resides in a locked box. Also in the dungeon are various keys, two of which are golden keys, and a key that looks like it is made out of demon blood like Finn's Demon Sword. One key unlocks a small treasure chest containing a stone heart, or possibly a stone butt with eyes, hidden in the dungeon.
The "Vault" itself is more akin to some sort of Crypt, due to the staggering multitudes of bones which lie scattered throughout the dungeon. However, Finn, in regard to these bones, simply states that the dungeon, "is trying too hard".