The Eye of Cthulhu Summoner, introduced by the Terraria Idea Pack, is crafted from one Fake Eye and one Diamond in a shapeless recipe. It can be right-clicked with to summon the Eye of Cthulhu. If the Eye of Cthulhu is already in the world, it will simply be consumed and not summon anything. If there is no Eye of Cthulhu active in the world, it will spawn one. If there is already an Eye of Cthulhu, it will not do anything and not be consumed.