Tissue Sample

Most of these come off the Creepers orbiting the Brain of Cthulhu rather than the boss itself, which should change how you fight it. Clearing the shell properly instead of rushing into the second phase is what fills your inventory, so a weapon that catches several Creepers per swing earns more here than raw single target damage. In Expert the treasure bag tops you up as well.

Budget for more than one fight. Crimson armor and the Deathbringer Pickaxe together take a serious pile, and the pickaxe comes first because it is what lets you touch Hellstone. The Void Bag comes back for another chunk much later in a run, so hold the extras instead of selling them off.

A Crimson world will never grow the Corruption material on its own. Shimmer is how you convert if a recipe asks for Shadow Scales.