Brain of Cthulhu

. The Brain of Cthulhu is the Boss of the Crimson biome, appearing every time a player shatters three Demon Hearts or uses a Bloody Spine. It replaces the Eater of Worlds, and can only be summoned in the Crimson using the Bloody Spine.

Behavior
There are two distinct phases in this boss battle. In the first phase, the Brain will be invincible and teleport around the area, surrounded by eyes called Creepers that attempt to fly at the player, dealing damage. Each Creeper has its own health bar, and will drop Tissue Samples, Crimtane Ore and hearts. When all the eyes are destroyed, the Brain itself is able to be damaged normally. In the second phase of the fight, the Brain will continue to teleport, but will concentrate on ramming the player, and moves faster than its first form. In both phases, touching the Brain itself will deal high damage to the player.

Tips

 * Similar to the Eye of Cthulhu and Eater of Worlds, building an 'arena' consisting of long stretches of Wood Platforms allows the player to maneuver around the Brain itself, and the eyes of the first stage.
 * The Flare Gun is good at dispatching the eyes in the first phase, as when they are hit they will retreat back to the Brain, and the fire will eventually finish them off.
 * Shurikens are also a good way to dispose of the eyes in the first phase for new players. However, they have next to no knockback, and cannot stop the Brain's charges towards the player in the second phase.
 * The Harpoon's knockback works great during it's second form.
 * Gem staffs, like the Diamond Staff, can combo and make short work of the eyes on open spaces while saving ammo from other ranged weapons, especially if the staff has a good modifier.
 * During the second phase, it's a good idea to have a weapon with either a lot of knockback, or multiple hits with average knockback (such as a Spear). This will allow the player to keep the Brain at bay as it attempts to charge into them.
 * Using the Crimson Rod, which has a chance to drop from one of the Demon Hearts required to spawn the Brain, can provide extra DPS, especially against the eyes. Fire it to the top-center of the arena then switch to another weapon for maximum damage.
 * A Vilethorn is extremely useful against the eyes, as its tendency to stun-lock enemies prevents the eyes from retreating when the brain teleports away. However, the Vilethorn is only available from Shadow Orbs in the Corruption, and therefore will need to be brought over from another world.
 * If the player has had luck getting meteorite spawns, the Space Gun makes short work of its first stage, as any given shot is likely to hit multiple eyes. Meteor Armor is then a good option as it ensures mana is not an issue.
 * If a Demolitionist has moved into the players world, Grenades will also make short work of the eyes. Throwing a couple when they group together can kill or majorly wound them all with one blast.
 * If the player has obtained a Beegun from the Queen Bee, the first stage can go smoothly so long as you have the mana for continuous fire and are able to grab the stars as the eyes die off. In the second stage, it can help to use a short burst to keep up damage and track the Brain.
 * If the player has crafted a gem staff, it can be used as a viable weapon against the eyes, due to its piercing quality. In the second stage, it can be used to knock back the Brain enough to stop its charge attack, but may have limited use due to the lack of mana-generating eyes.
 * If a player has obtained the Blade of Grass and a Boomstick from the Underground Jungle, the fight against Brain of Cthulhu can be made very easy. Strike the creepers with the blade until they are destroyed, and then keep suppressing fire from the Boomstick on the Brain.
 * Jester Arrows are insanely effective against his minions, leaving about 2 to 3 left in just a few seconds.
 * If you manage to get the Demon Scythe item and plenty of Mana before actually fighting the Brain, it is extremely useful. Seeing as each of the Demon Scythe's attacks can hit most of the Eyes for a good deal of damage, and be able to hurt the Brain rather rapidly seeing as it likes to get close for attacking, leaving it possible to leave a Scythe to charge and deal even more damage.

Trivia

 * The Brain of Cthulhu, along with the Eye of Cthulhu, is based on the character Cthulhu from the works of the author H.P. Lovecraft.
 * The Brain of Cthulhu strongly resembles the boss Phantoon from the game Super Metroid, both in appearance and behaviour.
 * The Brain of Cthulhu's second form appears to have a heart inside of it, as well as an eye in the heart.
 * The Brain of Cthulhu is the only boss susceptible to knockback.
 * Unlike the Eater of Worlds, the Brain of Cthulhu doesn't run away if you leave the Crimson.
 * The Brain of Cthulhu, the Eater of Worlds, Queen Bee, the Wall of Flesh, Golem, and Plantera are the only bosses that do not immediately go off screen and despawn after you have died.
 * After killing The Brain Of Cthulhu, the heart appears to have a gash just below the eye, resulting in a sort of unhappy face.