Corrupted - 10G - Complete the Dalish Elf origin story
For this origin story it is really straight forward, you will stop some running humans who talk about a cave, go there, the only enemies encountered on the way and in the cave are wolves, giants spiders and i think a few darkspawn, just fight your way through.
After the events of the cave talk to your clan leader and she will then tell you to re-enter the cave with a girl at the bottom of the camp, go back into the cave after more cut scenes you'll be back in the camp and you will be asked to become a grey warden by Duncan.
On the loading screen to Ostagar you should get the achievement.
(I didn't want to give too much of the origin story away so if it sounds bad in places leave a comment and i'll edit it.)