The Suspicious Looking Eye is used to summon the Eye of Cthulhu boss. It can be crafted at a Demon Altar or Crimson Altar at the cost of 6 Lens. It can also be found rarely in Golden Chests. The boss can only be successfully summoned at Night; if the eye is used during the Day nothing will happen and it will not be consumed.
Using a Suspicious Looking Eye while the Eye of Cthulhu has already been summoned by another Suspicious Looking Eye will result in the Player's immediate death.
Using the Suspicious Looking Eye during a natural occurrence of the Eye of Cthulhu (1:3 chance every night after the conditions are filled) will result in fighting up to two Eyes of Cthulhu simultaneously.