Brain of Cthulhu

The Brain of Cthulhu is a pre-Hardmode Crimson boss that can be summoned by smashing Crimson Hearts. It is one of the two world-specific evil bosses, serving the Crimson counterpart to the Eater of Worlds. Defeating it is the only way to unlock the Tavernkeep and Meteorites, and in Expert Mode it is also the only way to obtain the Brain of Confusion.

The battle has two distinct phases. In the first phase, the Brain itself is invulnerable and teleports randomly around the player while 20 Creepers orbit it at high speed. These Creepers cannot respawn once killed, and the Brain advances to its second phase only after all 20 have been defeated. In the second phase, the Brain becomes directly vulnerable and can be killed, but it also begins using illusion copies to confuse the player.

Several practical details make the fight much easier. Creepers have low individual health and are especially vulnerable to On Fire! and Frostburn, while splash weapons such as Grenades can clear clustered groups quickly. Piercing weapons like Jester's Arrows are very effective in the first phase because the Creepers often bunch together, allowing one shot to hit multiple targets. The Brain itself is susceptible to knockback, so high-knockback weapons can help keep it and the Creepers from closing in between teleports. Spears are especially useful for this reason, with the Trident and The Rotted Fork being strong choices, and early melee weapons such as the Katana or the Zombie Arm can also work if their modifiers are not poor. The Minishark is also highly effective at keeping distance during the second phase.

There are multiple ways to identify the real Brain in phase two:

• It is the only one with a health bar.
• Bone Javelins stick out only from the real Brain.
• Minions and sentries target only the real Brain.
• On Fire!, Frostburn, and Poisoned damage only the real Brain and show their visual effects on it.
• The Slime Gun and Water Gun only apply visual effects to the real Brain, though they cannot deal damage.
• The Slime Mount only bounces off the real Brain.
• The illusions move relative to the player; if the player moves away, the illusions spread out quickly while the real Brain keeps moving at a steady pace.
• On controller, lock-on targets only the real Brain.

The fight is also notable for its rewards and progression quirks. Creepers drop Hearts at a higher rate than most enemies, but they do not drop Souls of Night or Biome Keys. Because the Creepers can drop much of the useful Crimson material, fully defeating the boss is not always necessary for progression; however, the boss must be defeated to gain access to the Tavernkeep and Meteorites, and in Expert Mode the actual Brain must be defeated to obtain Brain of Confusion. Selling the Crimtane Ore earned from the fight, or converting it into Crimtane Bars first, is an effective early-game money source.

A few other details are worth noting. Leaving the Crimson causes the Brain of Cthulhu to move downward and despawn, though returning before it disappears will continue the fight normally. Two Brains can be summoned at once by smashing multiple Crimson Hearts, and they share their Creepers and enter phase two together. In special worlds such as For the Worthy and Zenith, it gains increased damage and defense, spawns twice as many Creepers, moves faster in its first phase, uses fully opaque illusions in its second phase, and has gravity reversed once phase two begins.

The Brain of Cthulhu is one of the least durable bosses in the game when considered by itself, though its Creepers add substantial total health. It is also the only boss affected by knockback, and its theme is shared with the Frost Legion. A cloud shaped like the Brain of Cthulhu can occasionally appear in the background.