You need to go the evil route on the Urn of Sacred Ashes quest, right at the end. Agree to do Kolgrim's dirty work, then, when you're in the temple, pour blood on the ashes to ruin them. When you come back outside you'll receive the specialization.
One thing, though: if you do this most people in the group will hate you, and if Leliana and/or Wynne are in your group at the time they'll attack, and you'll have to kill them off. If they aren't, Wynne will leave the group when you next enter camp. (Leliana won't, however.)