Crimson Chest

The Crimson Chest will only be in the dungeon if crimson has replaced corruption. Otherwise the corruption chest will be there. You will never have one of both of the same world. It has a 100% chance to contain the Vampire Knives, which arguably is the best melee weapon in the game. The chest is opened with a Crimson Key which is dropped from a random Crimson enemy in the Crimson when hard mode is enabled. The Crimson has a chance to replace the Corruption when generating a new world.

Tips

 * Before enabling hard mode make sure you find the location of the chest in the dungeon as hard mode makes the dungeon significantly harder.
 * You can open the chests on a non HardMode world.