Towards the end of The Void map you will find Clown Key.
This is used to unlock the iron bar gate you saw at the start of the void and leads to a whole new section of the map.
At the end you will find Dungeon of the Decayed. This is a gauntlet style boss rush where you fight the redux version of the various bosses you encountered back to back with a few new fights mixed in.
You will get a new accessory, plenty of mana and gold, as well as The Sword of Deception (800ATK) and The Sword of Verum (400ATK) now while it has less attack than The Sword of Deception, it is still the best sword in the game. The Sword of Verum has a Very Fast swing speed while the later is Very Slow, put some good runes on Verum and you can rapidly kill any boss easy.
Finishing the dungeon and getting the sword will net you the Achievement.