Brain of Cthulhu

The Brain of Cthulhu is a pre-Hardmode boss that spawns in or is summoned within the Crimson by smashing Crimson Hearts. Its encounter is a two-stage fight involving the Brain itself and a set of smaller orbiting enemies called Creepers. The Brain teleports and produces illusions during the first stage and becomes vulnerable only after all Creepers are destroyed. It drops Crimtane-related materials and is required to unlock certain NPCs and items; defeating the Brain is the canonical way to progress past its content.

The battle begins with the Brain immune to damage while it teleports around the player and twenty Creepers spawn to orbit and harry the player. Creepers move quickly and frequently group together; they cannot respawn once killed, and the Brain becomes vulnerable only when all Creepers are defeated, entering the second phase in which it can be directly damaged and defeated. The Brain is unique among bosses in that it is affected by knockback and in its second phase may create illusory copies that complicate targeting. If the player leaves the Crimson, the Brain will descend and despawn unless the player returns before the despawn finishes. Smashing multiple Crimson Hearts can summon two Brains simultaneously; paired Brains appear to share Creepers and enter their second phases at the same time.

Practical strategies and interactions:

• Focus Creepers first in stage one. Creepers have low health, drop Hearts at an increased rate, and are especially susceptible to On Fire!, Frostburn, and Poisoned debuffs; weapons or ammunition that apply these debuffs speed the clear. Splash-damage weapons such as grenades are effective because Creepers bunch up often.
• Piercing weapons and projectiles (for example, Jester's Arrows) are useful in stage one because they can hit multiple Creepers when they cluster.
• High-knockback weapons greatly help keep the Brain and Creepers from closing distance when the Brain teleports. Spears and high-knockback melee weapons (Trident, The Rotted Fork) are recommended; early-game melee weapons like the Katana or Zombie Arm can also work if modifiers are favorable.
• During the second stage the Brain becomes vulnerable but creates illusions. Several guaranteed methods reveal the real Brain: only the real Brain displays a health bar; only it can be damaged by On Fire!, Frostburn, or Poisoned and will show those visual effects; Bone Javelins will stick only in the real Brain; minions and sentries will target it; the Slime Gun/Water Gun apply visible effects only to the real Brain; the Slime Mount bounces off the real Brain; controller lock-on targets only the real Brain. Minishark and other rapid ranged weapons help kite and control the Brain in stage two.
• In Expert Mode and Master Mode the illusions make distinguishing the real Brain harder, so rely on the listed mechanical indicators (health bar, minion targeting, status-effect visuals).
• Post-fight rewards: selling Crimtane Ore or Crimtane Bars obtained during the fight is a reliable pre-Hardmode income source. Defeating the Brain is necessary to recruit the Tavernkeep NPC and to enable Meteorite to fall in the world. In Expert Mode the Brain must be actually defeated to obtain the Brain of Confusion accessory.
• Variants and special-world changes: in For the Worthy and Zenith special worlds the Brain has increased damage and defense, spawns 40 Creepers, and exhibits faster movement and charging patterns; gravity may be reversed during its second phase in those worlds.

Trivia and notable facts:

• The Brain of Cthulhu is based on the literary creature Cthulhu. It has the least base boss health among bosses in its vulnerable form, though when combined with the Creepers' collective health it surpasses some other early bosses. Its music is shared with the Frost Legion and its map icon remains unchanged after entering the second phase. The Brain is commonly abbreviated as BoC.