Brain of Cthulhu

The Brain of Cthulhu is a pre-Hardmode Crimson-themed boss resembling a large, flying brain. It can be summoned when players destroy 3 Crimson Hearts in a Crimson world, or when a player uses a Bloody Spine anywhere in a Crimson biome.

The Brain of Cthulhu has two stages. In the first, the Brain is invulnerable and teleports randomly around the player. 20 Creepers spawn along with the Brain and orbit the boss at high speed. Creepers do not respawn once they are killed. All Creepers must be destroyed first before the Brain of Cthulhu enters its second phase, where it becomes vulnerable to attack and can be killed.

Defeating the Brain of Cthulhu and its attendant Creepers is the only way to obtain Tissue Samples, and is one of the ways to obtain usable quantities of Crimtane Ore. Defeating the Brain of Cthulhu for the first time allows the Tavernkeep NPC to spawn.

The Brain of Cthulhu is the Crimson counterpart to the Eater of Worlds. Unlike its Corruption counterpart, it does not have a mechanical equivalent in Hardmode.

Its Bestiary entry reads: "A piece of Cthulhu torn asunder, this vile mastermind pulses with agony and aids the Crimson in an attempt to avenge its master."

First phase
The Brain itself is initially invincible. It teleports around the player, drawing nearer before teleporting away again in a random direction while its attack the player. The Creepers do not teleport along with the Brain but are instead pulled between the player and the Brain in large, sweeping orbits. Once all Creepers are defeated, the Brain enters its second phase.

Second phase
In the second phase, the Brain becomes vulnerable to attacks and knockback and it aggressively attempts to ram the player before teleporting away.

In Expert mode, the second phase now has three mirror images of the Brain appear around the player, mirroring the Brain's movements and becoming more opaque as the boss loses health. Furthermore, both the Creepers (in the first phase) and the Brain (in the second phase) inflict a random debuff upon contact. The probabilities of applying each debuff are listed in the following table; note that the Creepers always inflict a debuff while the Brain only has a chance to do so.

In Master mode, the Brain of Cthulhu gains a significant health increase of roughly 27.5%. This brings its HP to 2167, an increase of 467 health points from its Expert mode equivalent. When factoring in the Creepers, the boss's overall health ranges from 6487 HP to 8647 HP. Other Expert mode elements apply.

Tips
The Quad Barrel Shotgun is an excellent weapon to use for the brain's second form. It deals extremely high close-range damage and can end the fight before the Brain’s illusions get too opaque.
 * Creepers have low individual health and are especially susceptible to the On Fire! and Frostburn debuffs. Weapons that inflict these debuffs can make quick work of them.
 * Since the Brain of Cthulhu is susceptible to knockback, weapons with high knockback such as the The Rotted Fork can keep both Creepers and the Brain from closing distance between teleports. It is also possible to use early-game melee weapons such as the Katana or the Zombie Arm as long as they do not have disadvantageous modifiers. It is preferable to use spears for this fight since they deal more knockback, however, so the Trident or The Rotted Fork is recommended.
 * Note that this may not work in, since it might be hard to tell which Brain to attack among the illusions.
 * Jester's Arrows or other enemy-piercing weapons can be immensely helpful while fighting the first stage, since Creepers often group up in a clustered formation, effectively allowing the player to hit most if not all of the Creepers in one well-aimed shot.
 * There are several methods to reveal the real Brain of Cthulhu in the second stage:
 * Bone Javelins only stick out of the real Brain.
 * The On Fire! and Poisoned debuffs damage only the real Brain.
 * The Slime Gun and Water Gun cause visual effects only to the real Brain. Note that these do not deal damage.
 * The Slime Mount only bounces off the real Brain.
 * The illusions position themselves in relation to the player. The illusions will quickly spread away from the real Brain if the player is walking away from it, but the real Brain will continue to move at a steady pace.
 * The minimap displays only the real Brain.
 * When using a controller, the lock on feature will only target the real Brain.
 * Selling all the Crimtane Ore that can be obtained from the fight is a very effective way to make money in pre-Hardmode.

Trivia

 * The Brain of Cthulhu, among several other bosses, is based on Cthulhu, a creature first imagined by in his short story .
 * Unlike the correlation between the Eater of Worlds and The Destroyer, the Brain of Cthulhu does not have a Hardmode counterpart, although The Destroyer's Probes are similar in function to its Creepers.
 * The Brain of Cthulhu has the least health out of all the bosses, with even less than the Eye of Cthulhu in its second form. However, when adding in the health of the Creepers, it has more health than King Slime.
 * The Brain is the only boss that is affected by knockback.
 * Contrary to popular belief, Creepers do not drop Souls of Night, Biome Keys, or  Biome Key Molds.
 * It is one of the only two bosses whose map icon does not change after entering its second phase, the other being Santa-NK1.
 * The Brain of Cthulhu is sometimes abbreviated as "BoC."
 * The list of debuffs inflicted by the Brain and Creepers in Expert Mode just so happens to be exactly the same as the list of debuff immunities granted by the Ankh Charm. The Ankh Shield also gives these immunities, with the addition of Burning and Chilled. Of course, it is fairly trivial to defeat the Brain of Cthulhu at the point in the game that the Ankh Charm can be obtained.
 * The list of debuffs inflicted by the Brain and Creepers in Expert Mode just so happens to be exactly the same as the list of debuff immunities granted by the Ankh Charm. The Ankh Shield also gives these immunities, with the addition of Burning and Chilled. Of course, it is fairly trivial to defeat the Brain of Cthulhu at the point in the game that the Ankh Charm can be obtained.