Tainted Blood

Tainted Blood is a quest that is part of the joining ritual of the Grey Wardens. You have to gather three flasks of Darkspawn blood to complete it.

Background
In order to join the Wardens, you need to drink darkspawn blood, which Duncan wants you to find in the wilds.

Walkthrough
Leave Ostagar and enter the Korcari Wilds. As you progress through the area, you should run across several Genlocks and Hurlocks. Just search their bodies for the blood.

Result
If you also finished The Warden Cache, you may join them.