Slime Crown

The Slime Crown is used to summon King Slime, a Boss-type enemy. While King Slime can spawn randomly at any time without the use of a summoning item (though only in the outer two-thirds of the map, laterally), the Slime Crown summons King Slime instantly, in any location. Trying to summon a second King Slime while one is already present will instantly kill the player. The Slime Crown can only be crafted at a Crimson Altar or Demon Altar.

Trivia

 * It resembles the King Slime monster from Dragon Quest.