Optional quest in the circle. Throughout the tower, you read notes that the apprentices have left. There is a floor with several activatable statues, but I can't remember which one. Activate the statue holding the bowl, then the statue with the sword upraised, and finally the statue with spear, flanked by other statues holding shields (in another room). Then go down to the room where you first meet Wynne and try to open the door going downwards. You will fight a nasty demon that drops yusaris, a pretty awesome dragon-killing greatsword.
See the Dragon Age wiki walkthrough Watchguard of the Reaching for more info.