This is unlocked by finding all seven soul shards inside the Myserious Union to resurrect Claves. Here are their locations:
- Level 1 - In a chest.
- Level 2 - A boss on the teleporter to level 3.
- Level 3 - In a chest.
- Level 6 - You need to buy this for 99,999,999 gold. To get this amount, kill the black and orange guys on level 12. They drop three million every fight.
- Level 8 - In a chest.
- Level 11 - A boss on the teleporter to level 13.
- Level 13 (Final) - A boss. You must have the other 6 shards to fight him.
Check this thread
on our forums for a decent map to the dungeon.