Mechanical Eye

The  is a consumable item that summons the boss The Twins. To craft this you need to combine 3 Lenses, 5 Lead Bars/5 Iron Bars, 5 Copper Bars and 7 Souls of Light at a Mythril Anvil.

The Mechanical Eye will summon The Twins anywhere in both pre-hard mode and hard mode worlds, but must be used at Night.

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Bugs

 * The Twins are sometimes known to not appear after being summoned. (This is a very common bug on Multiplayer servers.)
 * On rare occasions if The Twins are spawned using the Mechanical Eye and the player teleports far enough from the boss only one of the "Eyes" may return thus leaving only one of them to fight against.