Revenant

Revenants are powerful desire or pride demons inhabiting the corpses of the fallen, and as such, share the same strengths and weaknesses as creatures like devouring corpses, and fanged skeletons.

Dragon Age: Origins
Eleven revenants can be found during the campaign, as a part of four separate quests:
 * The Black Vials: Six boss-level Revenants are trapped in glass phylacteries:
 * Caridin's Cross, Denerim, Circle Tower, Orzammar Royal Palace, the Lower Ruins, and the Werewolf Lair.
 * The Mage's Treasure: Three boss-level Revenants in the Brecilian Forest, one in the west & two in the east. These are summoned by triggering gravestones.
 * Arl of Redcliffe: A lieutenant-level Revenant is found in the Redcliffe Castle courtyard when The Warden breaks in to investigate.
 * Unbound: Elite Boss Gaxkang the Unbound, who is found in Quaint Hovel (Dirty Back Alley in Denerim), switches between two forms: a revenant and an arcane horror.

Dragon Age: Origins - Awakening
It is possible to encounter Revenants in the Blackmarsh after escaping the Fade. The various Fade Portals spawn various undead and demonic enemies, including Revenants, though at this point in the game they are only Elite level and far easier to dispatch of than their boss-level, Origins counterparts.

Dragon Age II
Miscellaneous revenants can be found during the game, including: If Aveline is in the party during the "Mark of the Assassin" quest, there is a possibility of unleashing several phylactery-bound revenants inside Chateau Haine during her personal quest.
 * One on Sundermount during Act 1
 * One beneath Harimann Estate, during Repentance
 * Also in Act three, especially during the "Last Straw", Revenants can be encountered quite frequently during almost every battle involving the undead

Skills
Revenants have five different skills, and like party members, have conditions for their Combat Tactics:

Other statistics
Revenants have the following characteristics:
 * High Armor, Health, and Regeneration.
 * Immunity to Stunning and knockdowns.
 * Immunity to Cold and Nature damage, although they can be frozen.
 * Vulnerability to Fire damage; normal reaction to Spirit, Lighting , and (despite the Codex text) Physical damage.
 * Highly resistant or immune to flanking and backstabbing.

Some of their basic strategies involve:
 * Pulling or Mass Pulling ranged attackers and healers.
 * Damaging multiple opponents in melee.
 * Killing weaker party members in one blow (spiking).
 * Wearing down the party with their allies (skeletons or lesser darkspawn).