The Dark Sanctum Achievement in Fable 3 is a practical and fun one to unlock, and you need to do it to get The Tenderizer, one of the better legendary weapons. Watch this video for a walkthrough of how to get the Dark Sanctum, starting from the quest called "Leverage"
From the Creator:
Before you do this you need to do three quests.
1st Quest: You Need to do the quest "Peace, Love, and Homicide."
2nd Quest: Later in the game, Lesley will ask you for 5,000 gold so he and some other people can get the tools to move the boulders out of the entrance of the Dark Sanctum. The Quest is called "Excavation."
3rd Quest: Later into the game, you will have to defend the Dark Sanctum from a bunch of Hollowmen. The quest is called "Awakening.".
After these 3 quests, you will unlock the quest that I am doing in this video. This quest is called "Leverage."