In this final battle you’ll need to take on your old master. Head towards the fortress. You’ll be attacked by some of your fellow Assassins. If you are low on health disengage from battle and run around until your health is restored.
When you enter the garden Al Mualim will trap you. While you are trapped all of your past targets will attach you at once. Once you have killed them you’ll speak with Al Mualim again.
Once again Al Maulim will attack you after cloning himself many times. You can see the real one with Eagle Vision but switching targets mid-battle is almost impossible so just take out the nearest target until you kill him.
Finally Al Mualim will attack you for real. Once you hit him he will draw away all of your health except for one bar and teleport to another area of the gardens. Search around to find him and throw daggers at him before engaging in battle once more. Continue until he’s been defeated.
After completing the Memory you’ll be pulled back into real time but now you have the ability to us Eagle Vision. Head into your bedroom and look at the writing on the wall to complete the game.