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 Lenses. It can also be found in Golden Chests rarely. 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 will not be consumed and won't summon another Eye of Cthulhu
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.
The Suspicious Looking Eye is one of the most readily avaliable summon item to players, as it is relatively easy to craft and can be found in Chests. However, as the required materials to craft it (Lenses) are only avaliable during night, the Worm Food is more readily available to players in a Corruption world, and the Bloody Spine to players in a Crimson world.