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.

another method is to gather traps from the lihzard temple (namely spikey ball and spear) and set them up like this