This dungeon will be available after you finish the game at least once. Before going to Zaude on your first play of the game, revisit Phaeroh's Crag to find a rift that will take you back to the illusion of Yormgen. Talk to the Sage and agree to sleep in his house. From there, go ahead and continue to play and beat the game, then save in a new slot. Once you load that save (it will have a star on it now) you will be just before the final boss but it will have recognized that you beat the game. You can continue working on side-quests and this dungeon will now be available by returning to the illusion and getting a book from the Sage. From there, visit Dahngrest for a scene and then fly west toward Yumanju for another scene. Enter the glowing orb on the islands south of Yumanju to access the dungeon.
To actually clear the dungeon you will need 15 Fake Gald to enter the final room and face off against the Traitor to Heaven. Once you have defeated him you will leave the dungeon, get a short scene and the achievement. To get the Fake Gald, open all the treasure chests (which will be locked until all enemies in the room are killed) and kill bosses. It may take a few runs of the dungeon to get the 15 Fake Gald needed. You may sometimes only get one or two, other times four or five.