Mechanical Skull

The Mechanical Skull is used to summon Skeletron Prime, the hardmode version of Skeletron. It can only be used during night time. Using another Mechanical Skull while Skeletron Prime is alive will not consume the skull, and a second Skeletron Prime will not be summoned. It can also be used in a world without hardmode activated. If you have not defeated Skeletron Prime yet and are already in hardmode, all enemies (with the exception of statue-spawned enemies and meteor heads) have a 1 in 2500 chance of dropping this item.

Trivia

 * The Mechanical Skull bears a slight resemblance to Skeletron's pre-1.0.4 sprite.
 * Like the Mechanical Worm, it looks like a wind-up toy.

History
Crâne Mécanique 기계식 두개골