Reaver (Origins)

Demons can teach more than just blood magic. Reavers terrorize their enemies, feast upon the souls of their slain opponents to heal their own flesh, and can unleash a blood frenzy that makes them more powerful as they come nearer to their own deaths. This is a specialization of the warrior class. You can learn it by contaminating the Urn of Sacred Ashes and talking to the leader of the cult afterward.

Reavers in Thedas
Kolgrim

Cultist Reaver

Reaver: The Class
Specialization benefits for Reaver: +1 constitution +5 physical stamina.

Devour
The reaver revels in death, absorbing the lingering energy of all nearby corpses, each of which partially restores the reaver’s own health.
 * Required Level: 7
 * Stamina Cost: 25
 * Upkeep: 0
 * Fatigue: 0
 * Cooldown (sec): 30
 * Area of Effect (ft): 5

Frightening Appearance
This talent focuses the reaver’s unsettling countenance into a weapon, making a target cower in fear unless it passes a mental resistance check. Frightening Appearance also increases the effectiveness of Taunt and Threaten.
 * Required Level: 12
 * Stamina Cost: 25
 * Upkeep: 0
 * Fatigue: 0
 * Cooldown (sec): 20
 * Area of Effect (ft): 0

Aura of Pain
Radiating an aura of psychic pain, the reaver takes constant spirit damage while this mode is active, as do all enemies nearby.
 * Required Level: 14
 * Stamina Cost: 0
 * Upkeep: 60
 * Fatigue: 5
 * Cooldown (sec): 45
 * Area of Effect (ft): 4

Blood Frenzy
Driven by pain, the reaver gains larger bonuses to damage whenever health decreases. Because this mode also incurs a penalty to health regeneration, the reaver flirts with death the longer the frenzy persists.
 * Required Level: 16
 * Stamina Cost: 0
 * Upkeep: 60
 * Fatigue: 5
 * Cooldown (sec): 60
 * Area of Effect (ft): 0

Unlocking
At the end of the Urn of Andraste quest, pour the blood into the urn and speak with Kolgrim outside. He will teach you the Reaver specialization.

Its is recommended that you do not have Wynne or Leliana in the party when fouling the ashes as they will automatically attack you, as the guardian also attacks you it can be an incredibly hard fight, especially as you will only have yourself and one party member remaining. So other party members should be taken rather than them.

If you do not wish to suffer the repercussions of fouling Andraste's ashes, such as losing Wynne permanently (regardless of whether or not she was in the party when you completed the quest in that fashion) you can save before speaking to Kolgrim the first time, complete the quest, and back up to the previous save. Reaver will still be unlocked for that profile.

Fouling Andraste's ashes with the blood will not automatically cause Leliana to leave the party. You will need to persuade/lie to her, which will work if she wasn't in the party when you completed the quest. She will just take an approval hit which is easily countered by gifts and more pious or charitable actions later.

Reference
Dragon Age: Origins - PRIMA Official Strategy Guide