Mastermind

The Crimson's boss. Summon by smashing three Crimson Hearts (Smashing, Poppet!) or crafting a Bloody Spine (15 vertebrae at a Crimson altar). Phase one: pop the floating Creepers - piercing weapons shine; phase two: the exposed brain teleports and charges, so keep moving and keep hitting. Fight in the Crimson's own arena-sized caverns, cleared of obstructions. Crimtane + tissue samples from the fight build your next gear tier. Corruption worlds get Worm Fodder instead - world evils are one-per-world, so the other means a second world (or a Corruption/Crimson seed trick).