Crimson Key Mold

The Crimson Key Mold is a rare Hardmode drop used to craft the Crimson Key, which is in turn used to unlock the Crimson Chest in the Dungeon. The mold can be found from any monster in the Crimson and Underground Crimson biome with a 1/2500 drop rate, or a 0.04% chance, after the world has entered Hardmode (ie. you must kill the Wall of Flesh before you can farm for the mold).

Tips

 * A good way to farm for the key mold is to farm the Brain of Cthulhu, because the Creepers count as kills.
 * Another way to farm the mold is to clear out a large cube-like space in the underground Crimson, and place a block that is the height that the monsters are off your screen, and grapple onto it and attack from there using a ranged weapon. If they are off your screen, it wont affect the spawn rate of the monsters. A Water Candle combined with Battle Potion can greatly increase the spawn rate. Any minons will help, also notifying you if there are monsters below you.
 * For farming you can make meteor heads spawn and kill them it will take a while.