The Shield of Cthulhu is an Expert Mode shield accessory obtained from the Treasure Bag dropped by the Eye of Cthulhu. Equipping it allows the player to perform a dash attack by double-tapping either the ◀ Left or ▶ Right key. Dashing into an enemy
destroying 3 crimson hearts will spawn Brain of Cthulhu which is the Crimson boss and the Crimson & it's Bosses doesn't have any connection with the Eye of Cthulhu, instead the Eye of Cthulhu will spawn randomly if these requirements are met The Eye of Cthulhu has not yet been defeated...