The Suspicious Looking Eye is used to summon the Eye of Cthulhu boss. It can be crafted at a Demon 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. Take note that if the Eye of Cthulhu isn't defeated by dawn it will disappear.
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 (Such as holding the Water Candle for long enough or killing 20+ Demon Eyes) will result in fighting up to four Eyes of Cthulhu simultaneously.