Main article: Bosses
The Brain of Cthulhu will appear after every third Crimson Heart is broken or after a Bloody Spine is used. Crimson Hearts are found in the Crimson’s chasms, surrounded by Crimstone, which can be bypassed with the same methods Ebonstone can. However, some of the pockets containing these will generate open. Once within range, Crimson Hearts can be destroyed with a Hammer or any tile-breaking explosive. It can only be summoned in the Crimson and can be at any time of day.
Brain of Cthulhu has only 1,000 HP (1,700 in Expert Mode), giving it the lowest amount of HP out of any boss. The Brain of Cthulhu has two forms. In the first form, the brain cannot be harmed, and Creepers will float around it. After all of these are killed, the Brain of Cthulhu will go into its second form, where it will teleport more frantically and move a lot faster, so a ranged weapon may be needed. The Brain of Cthulhu’s second attack form is ramming, alike the second form of Eye of Cthulhu. Unlike most other bosses, the Brain of Cthulhu is not immune to knockback.
The Brain of Cthulhu is the only source of Tissue Samples in the game (other than the creepers spawned while fighting it), and drops a large amount of Crimtane Ore, which are their equivalents to the corruption items.
In Expert Mode, the Brain of Cthulhu will have more health, and in its second phase, it will start creating illusions of itself to charge at the player. The lower the Brain of Cthulhu’s HP gets, the illusions become much less obvious to the point where they will look exactly like the real Brain of Cthulhu.