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From Terraria Wiki
The 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, which means consuming this item during day will not spawn The Twins. Using another Mechanical Eye while the previous Twins are active will not consume the Mechanical Eye, and a new pair of Twins will not be summoned. It can also be used in a world without Hardmode enabled. If you have not defeated The Twins before dawn, they will flee and the player will not be able to finish defeating them.
If you have not defeated The Twins yet and are already in Hardmode, all enemies (with the exception of statue-spawned enemies and Meteor Heads) have a 1/2500 chance of dropping this item.
- The Mechanical Eye is the only mechanical item with no visible winder.
- 1.3.1: Sprite updated. Old sprite was .
- 1.2.1: Drop chance now reduced from 1 in 1000 to 1 in 2500.