Crimson Key

The Crimson Key is a 1/4000 drop from enemies found in the Crimson after a world has entered Hardmode. It is used to unlock the Crimson Chest found in the Dungeon.

Usable farming method: get a Pygmy Staff, then go farm 1.5 platinum and buy the Tiki armor, and Pygmy Necklace from the Witch Doctor, then hang from a block ~15 blocks from a platform in the large center cavern as picture below. Reliable, semi-safe (you'll only occasionally get hit by the occasional herpling jump).

Alternatively; put 50 meteor blocks under you and farm meteor heads.