Mechanical Eye

is used to summon The Twins, the advanced and Hardmode version of the Eye of Cthulhu. It can only be used at night; using this item during the day will not spawn The Twins nor consume it. Using another Mechanical Eye while the The Twins are active will not cause any additional effects, and will not be consumed. If The Twins have not been defeated yet in a Hardmode world, all enemies have a chance of dropping this item, with the exception of statue-spawned enemies, Meteor Heads, and Old One's Army enemies.

Trivia

 * The Mechanical Eye is the only mechanical item with no visible winder.