Note: If you want to get Traveler, you have to visit all locations on the Fade map.
To begin this quest, head to the Lake Calenhad Docks and take the ferry to Circle Tower. Once you get on the ferry, there's no turning back. You'll have to finish "Broken Circle". When you arrive at the tower, you'll find it in disarray and overrun by blood mages and demons. Fight your way to the top level of the Tower where you'll meet the Sloth Demon. On the first level of the Tower, you'll meet Wynne. You want her in your party. Here is where it gets interesting. You'll be sent into the Fade. There are a series of "stages" that you must complete. There are six with a demon that you must kill. Four of these also have dreaming Templars and Mages that will give you a Shapeshifting ability. After destroying these demons, you have to rescue the other three members of your party, then fight the Sloth Demon. Once you escape the Fade, then you have to face the blood mage that has overtaken the Tower. You'll meet an imprisoned Templar on the way up to the blood mage. He'll ask you to kill all of the Mages. Refuse him and elect to spare the mages.
During the fight, the blood mage will attempt to turn the other captured mages into sloth demons. If you don't use the Litany of Andraste to protect them, you could be unwittingly forced into Annulment Invoker if you're not careful. The Litany protects the mages and prevents the blood mage from corrupting them. Once the fight is done and you have rescued the Mages, return to the first floor of the Tower and say that the Mages are all safe. The Mages will then join your army as a result.