To Catch a Thief

Late in Act 2, return to the Hawke Estate to find Isabela and Aveline arguing, both needing your help. Isabela reveals that she has found the relic that she lost. This also means that her pursuer, Castillon, will spare her life if the relic is returned.

Acquisition
Enter the Hawke Estate at Hightown, and choose to help Isabela first to activate the quest.

Walkthrough
Head to your home in Hightown after leaving the Chantry. On entering your mansion, you will find Isabela and Aveline arguing. Once you've chosen to aid Isabela in reclaiming the relic, head to Lowtown (with Isabela in your party) to find Wall-Eyed Sam. Over at Lowtown, things begin to heat up with a fight against a Sten and a couple of Qunari. The cutscene reveals that the relic Isabela has stolen was a bible written by Ashkaari Koslun, who wrote the Qun. The Qunari have been unable to leave Kirkwall without the relic. At the end of this conversation, you must choose between helping Isabela, which gives  and, or returning the relic to the Qunari, which gives  and.

Enter the building for a fight with Tevinter Mages and a bunch of Qunari headed by a Sten. Isabela will run at the start of the battle, which leaves you and two other party members to plow through the remaining enemies (assuming you have kept your distance until the Qunari or the Tevinter Mages have killed each other). In the room to the north-west, there's an upgrade for Isabela's armour. Leave the building once you've looted everything.

Once you've left the building, you'll find Isabela's Note on Wall-Eyed Sam's body.

With that, this quest ends.

Result
Isabela will have left your party for good if she is not romanced, or you don't have a high enough friendship or rivalry.

Should you have a high enough friendship/rivalry, it will appear that she has backstabbed you and left the party for good, but she will return with the relic in the following quest - Demands of The Qun.

Rewards

 * Saartoh Bas-Vat Iss
 * Saartoh Bas-Kos Iss
 * Sata-Kas Iss
 * Gauntlets of the Ashaad
 * Isabela's armour upgrade Lambswool Insoles

Trivia

 * While Isabela is describing the relic she mentions one of the options as Cousland, which is the name of the human noble Grey Warden of Dragon Age: Origins.

Bugs
There is a chance that you don't have to fight the Sten and his Qunari at all. Just move straight towards the foundry entrance and click on it. You will be brought to a cutscene without having to battle.