During the quest "Sacred Ashes", side with Kolgrim, he will give you the reaver specialization as a reward.
You can then kill him to get his loot.
If you don't want to choose is path, you have to save before defiling the ashes. Then you defile them to unlock the reaver class. When you load your old savegame, it is still unlocked, as specialization stay unlocked, so you can just take a pinch and step away. Kolgrim will attack you when you come out of the gauntlet.