Dungeon Guardian

The  is an enemy that spawns if the player enters the dungeon in a world where Skeletron has not been killed. It will appear in order to kill the player as soon as the dungeon music starts. In addition, it will also target any other players in the vicinity of the player who entered the dungeon.

Due to its unattainably high attack and defense, the Dungeon Guardian will always take 1 damage from all attacks (except criticals, which deal 2 damage) and will kill any players in one hit.

History
Before the Dungeon Guardian existed, Skeletron Heads fulfilled a similar role. Unlike with the Dungeon Guardian, killing all of the Skeletron Heads after they appear will grant the player access to the Dungeon as if they had killed Skeletron normally. The Dungeon Guardian was presumably introduced to counteract this unintended behavior.