Main article: Bosses

Skeletron is summoned by speaking to the Old Man NPC at the entrance of the Dungeon and activating his curse at night. Skeletron will alternate between attacking the player with its hands and performing a spinning attack. Its hands can be attacked and defeated to make the battle easier, although this is not required. If it has not been defeated before the night is over, Skeletron will attack and kill the player in one hit, despawning afterwards.

Defeating Skeletron will stop the Dungeon Guardians from spawning, allowing free access to the Dungeon. It can be summoned again by killing the Clothier with the Clothier Voodoo Doll equipped at night.

In Expert Mode, the fight is much harder. In the first phase, Skeletron’s head is more resistant to damage until its hands are destroyed. Once they are destroyed, it will start shooting homing skulls at the player. Expert Mode Skeletron is much faster. Its hands inflict the Slow debuff, and its head inflicts Bleeding. If the player gets caught by its head-spin attack, Skeletron can “stun lock” them, and often kills very quickly unless the player can grapple out.