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
you can get using helpful farms such as pirate invasions