This quest is repeatable.
The expert assassins throughout the Assassin's Tower will have random disks in their corpse. You will need these disks for the puzzle cabinet at the top of the tower.
Possible disk colors from expert assassins:
The Grand Master Assassin also has a disk. He will always drop a smooth green disk. He is the only one who can drop this.
North from the room prior to the Grand Master Assassin, there is a cabinet.
If you look ceiling it will show a series of colors. You must put disk cabinet in this order based on the color of the disks.
Once you have put them all in the cabinet, turn crank and you will receive a cracked silver mirror!
Note: you may need to clear the Assassin's Tower multiple times to receive all of the disks you need