Brain of Cthulhu

The Brain of Cthulhu is a Crimson Boss that resembles a large flying brain. It spawns when players destroy 3 Crimson Hearts, or when a player uses a Bloody Spine.

20 to 30 Creepers spawn along with the Brain and orbit at high speed. All Creepers must be destroyed first: As long as even one survives, the Brain cannot be damaged. Once the Creepers are destroyed, the Brain itself becomes vulnerable to attacks. The Brain cannot spawn new Creepers.

The Brain of Cthulhu is the Crimson counterpart to the Corruption's Eater of Worlds. Much like its cousin, fighting it is essential for progression, as the Brain and its attendant Creepers are the only source of Tissue Samples and usable quantities of Crimtane Ore, which are required to craft advanced tools and armor. Note that, like with the Eater of Worlds, actually defeating the Brain of Cthulhu is not completely necessary, as long as you are able to destroy enough Creepers to get the materials you need.

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 Creeper minions attack. 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. In Expert Mode, Creepers can inflict various status effects.

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

Expert mode
Also in the second phase, 3 mirror images of the Brain appear to distract the player, and become more visible as the boss loses health. And just like the creepers in expert mode, The brain can also cause various status effects.

Tips

 * As Creepers drop Tissue Samples and Crimtane Ore fairly frequently, it's technically not necessary to defeat the Brain of Cthulhu to gain access to Crimson-tier equipment, only to kill a couple of Creepers and secure their drops.
 * 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.
 * The Brain of Cthulhu is the only boss susceptible to knockback. Weapons with high knockback can keep both Creepers and the Brain from closing distance between teleports.
 * Unlike the Eater of Worlds, the Brain of Cthulhu doesn't run away and despawn if you leave the Crimson, therefore you are able to summon it and lead it to a nearby arena in a less hilly biome (it will still despawn if you teleport home).

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.
 * Unlike the correlation between the Eater of Worlds and The Destroyer, the Brain of Cthulhu does not have a Hardmode counterpart.
 * The Brain of Cthulhu has the least HP out of all the bosses, with even less than the Eye of Cthulhu in its second form. However, if counting the health of its creepers, it has more health than King Slime.