The Bone Serpent is an enemy (strongest non-boss burrowing enemy, the only burrowing enemy stronger than the Bone Serpent is the Eater of Worlds) found in The Underworld that can move through walls. Every part of his body except the tail has 150 HP. The Tail has 200 HP. The Bone Serpent's HP is not shared between those parts. In order to kill him, you need to destroy at least one part of him. The Bone Serpent is immune to lava, like all burrowing enemies.