Brain of Cthulhu

The Brain of Cthulhu is a pre-Hardmode Crimson-themed boss resembling a large flying brain. It spawns 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. 20 Creepers spawn along with the initially invulnerable Brain in its first stage and orbit the boss at high speed, following it wherever the boss teleports to. 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 then becomes vulnerable to attacks.

Defeating the Brain of Cthulhu and its attendant Creepers is the only way to obtain Tissue Samples. It also drops large quantities of Crimtane Ore similar to Eye of Cthulhu.

The Brain of Cthulhu is the Crimson counterpart to the Corruption's Eater of Worlds. Unlike its corruption counterpart, it does not have a mechanical counterpart.

On the, defeating the Brain of Cthulhu for the first time allows the Tavernkeep NPC to spawn.

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.

Expert mode
In the second phase, three mirror images of the Brain appear and become more opaque as the boss loses health. Furthermore, both the Creepers (in the first phase) and the Brain (in the second phase) inflict one of the nine following debuffs upon hit:

Master Mode
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 the expert mode counterpart. If counting the creepers the overall health is 6487 HP - 8647 HP. Other expert mode elements apply.

Creepers therefore always inflict a debuff, while the Brain only has a chance to do so.

Tips

 * Creepers have low individual health and are especially susceptible to the On Fire! and Frostburn debuffs, so weapons that inflict those 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 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 a Trident or The Rotted Fork can be used. Note that this may not work in, because 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 of the Brain of Cthulhu because Creepers often line up in a relatively straight 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 you are walking away from it, but the real Brain will continue to move at a steady pace.
 * 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.
 * Minimap shows only the real Brain.

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, if adding 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 mostly 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. Having one prior to the fight is a rather valuable asset, should one be using endgame equipment in newly generated expert/master mode worlds.
 * 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. Having one prior to the fight is a rather valuable asset, should one be using endgame equipment in newly generated expert/master mode worlds.