Fable II Guide - Castle Fairfax
Invest the money in shops and real estate so you are generating as much money as possible, then work on getting five-star acclaim in all the jobs in the game, particularly serving drinks behind the various bars in the game. The money is to purchase the massive Castle Fairfax, which holds the last few secret items in the game, but to actually trigger the specific quest, sleep in Lucien's bed, if you can bear it. You will be awoken by bandits attacking from a tomb underneath the castle, so take out those in your room and nearby, before proceeding down into the tomb to finish them off.
While your luck with enemy-infested houses thus far hasn't been great, you need to defeat these enemies to get the rewards on offer. Head through the enemies after activating the lever when you enter the tomb and then through the left-most door. You will eventually come to a room with another orb in, so use whatever colour button it recommends to whack it around the room until it allows you into the next room. Open the chests and pick up the Sex-change Potion on the table. This allows you, as the name suggests, to permanently change your gender for the rest of the game. You have to make the decision right now, in this tomb, or the opportunity is gone. Happily though your castle is now bandit-free, and as owner of the prestigious property you receive 10,000 renown.
The next quest also requires some formalities to be filled before it is activated, namely living a double life with two spouses!