Brain of Cthulhu

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

Behavior
There are two distinct phases in this boss battle. In the first phase, the Brain will be invincible and teleport around the room, surrounded by eyes that attempt to fly at the player, dealing damage. Each eye has its own health bar, and will drop Tissue Samples and hearts. When all the eyes are destroyed, the Brain is able to be damaged normally. In this second phase, the brain will continue to teleport, but will concentrate on ramming the player itself. In this second stage, the brain moves much faster than the first stage. In both stages, touching the Brain itself will deal high damage. It is the Eater of Worlds for Crimson, and is not very difficult to defeat, similar to Eater of Worlds.

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 Brain of Cthulhu will leave if the player attempts to escape from the underground area. So it's best to find a wide open area from within the chasms of the Crimson to begin building.
 * The Flare Gun is good at dispatching the eyes in the first phase, as when hit they will retreat back to the brain, and the fire will eventually finish them off. It is possible to take zero damage using the flare gun in the first phase.
 * Shurikens are also 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.
 * 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.
 * Bringing a Vilethorn from a world with Corruption is extremely useful against the eyes, as its tendency to stun-lock enemies prevents the eyes from retreating when the brain teleports away.
 * If the player has had luck getting meteor 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.

Trivia

 * The Brain of Cthulhu, like its counterpart the Eye of Cthulhu, is based on the character Cthulhu from the works of the author H.P. Lovecraft.
 * The Brain of Cthulhu is one of the few bosses (if not the only boss) affected by knockback.