Brain of Cthulhu

"A horrible chill goes down your spine..."

The Brain of Cthulhu is a pre-Hardmode Crimson-themed boss. 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 boss itself resembles a large flying brain in its first stage. In this stage, 20 to 30 Creepers spawn along with the Brain 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.

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.

Unlike its corruption counterpart, Eater of Worlds, it does not have a mechanical counterpart.

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

While the Brain of Cthulhu is alive, music Boss 3 will play.

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 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 give the player certain debuffs.

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 such as Darkness, Silenced and Weak.

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.
 * The Brain of Cthulhu is the only boss 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. If you use a high fire rate weapon, like The Minishark, it seems to move at a very slow rate.
 * It is possible to use early game melee weapons such as the Katana or Zombie Arm as long as they don't have negative Modifiers, since the Brain takes knockback. It is preferrable to use spears for this fight since they deal more knockback, however, so a Trident or The Rotted Fork can be used. This may not work in Expert mode, however, 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.
 * Another good way to kill off the Creepers and chip away at the brain would be to use many Spikey Balls scattered around your arena.
 * Using ~25-40 Grenades can be a very easy way of defeating the boss. Grenades can be very effective against the Creepers.
 * If you can get a Water Bolt from the Dungeon, it allows for a fairly good advanced strategy: Build a closed box (about 15x15), making sure you can get into it quickly for the fight.  Once the Brain appears, spam the Water Bolt to fill the box with bouncing bolts, which will rapidly kill Creepers passing through (quickly triggering the Brain's second form), then keep the Brain mostly away from you by knocking it back it every time it comes through the box. Make sure you have enough Mana or a Mana Flower!
 * For Expert mode, Bone Javelins can be used to reveal the real brain, as they stick out of it.
 * The Bee armor set along with the Hornet staff and bee gun can make very short work of the brain even in expert mode.
 * Using the Slime Mount in order to bounce on top of the creepers and the brain is a very effective way of avoiding damage, especially in Expert mode. This also reveals the real brain in Expert mode, due to it not causing you to bounce off the fake brains.
 * Once in Hardmode you can farm the creepers for Souls of Night and the Crimson Key/Mold as each one has a chance to drop it.

Trivia

 * The Brain of Cthulhu and the Eye of Cthulhu are 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, although The Destroyer's Probes are similar in function to its Creepers.
 * 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 the creepers, it has more health than King Slime.

History
克苏鲁之脑 Brain of Cthulhu/vi