Mechanical Eye

The Mechanical Eye is used to summon the The Twins (It's like two Eye of Cthulhu) It can be crafted in hardmode or non-hardmode worlds, but you need hardmode versions of crafting stations. You can only use it during night.

If Summoned at high altitudes, the twins may not appear; if they spawn from above where the map ends they disappear (all monsters that can pass through blocks that exit the map disappear).