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 biome.

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 Creepers 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. However, the Crimson has much more open areas underground than the Corruption's Chasms, requiring less setup.
 * The Flaming Arrows are 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.
 * Gem staves, 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. 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.
 * 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.
 * The Harpoon's knockback works great during its second form.
 * The Chain Knife's long range and rapid speed make it useful against the second form.
 * 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.
 * Jester Arrows are insanely effective against his minions, leaving about 2 to 3 left in just a few seconds.
 * If you have a Vertebrae farm, you can easily farm for Crimson Key Molds.
 * A very good weapon that can be acquired by breaking a Demon Heart is The Rotted Fork.
 * A good way to finish off the Creepers in its first phase is to have a small pit in the center of your arena filled with Spiky Balls, or lava. The Creepers will take constant damage when in the pit.
 * An effective weapon is the Flamethrower, for both phases. As long as you have enough gel, it can usually kill it quite fast.
 * For farming purposes, use a Paladin's Hammer, preferably reforged. Line the Creepers up and shoot the hammer. If the shot does not kill all of them, it should on the return. During the second form, the stronger knockback should help.
 * Note that each of the Creepers are susceptible to the On Fire! debuff, making the Flare Gun a good choice for DPS.

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 behavior.
 * 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, probably because of the difficulty in dealing with it if it wasn't.
 * Unlike the Eater of Worlds, the Brain of Cthulhu doesn't run away if you leave the Crimson.
 * Unlike the Eater of Worlds, the Eye of Cthulhu and Skeletron, the Brain of Cthulhu does not have a Hardmode counterpart. (The others having The Destroyer, The Twins and Skeletron Prime respectively.)