Xebenkeck

"The blood feeds, the blood nourishes. In blood, the call is heard. In blood, the deal is made. My master bathed in a river of blood, Then the Great Xebenkeck came!"

- Dabbon Hait

"The first of the magus cast themselves deep in the Fade in search of answers and power, always power. They found the forbidden ones- Xebenkeck, Imshael, Gaxkang the Unbound, and The Formless One. Many conversations were had and much of the fabric of the world revealed. And thus the magic of blood was born."

- Unknown Mage

Xebenkeck is a Desire Demon and an adversary character connected to a quest in Dragon Age II. She is described as being an undying. She can be found in The Forgotten Lair at the end of the quest Forbidden Knowledge.

Involvement
Xebenkeck (also known as Xebenkeck the Undying) is the end encounter of the "Forbidden Knowledge" questline. In that quest five tomes are encountered. If destroyed (reading or taking the tomes stops you from completing this quest) a new location appears in Darktown for the final tome and an encounter with Xebenkeck. In current builds this final tome may be read and the quest still completed, giving the +2 attribute points (Tome of the Mortal Vessel) and acting as if all books were destroyed.

When the tome is read, Xebenkeck appears along with 2 rage demons and several abominations (Nightmare). This is an incredibly tough fight, since once one of the demons or Xebenkeck (who heals them) goes down two more rage demons, a revenant, and several more abominations spawn in the room with the tome (Nightmare) in addition to a Revenant. Only by defeating all of them is the "Exorcist" achievement or trophy given, and some treasure appears in the room. Xebenkeck also drops the staff Voracity, a powerful fire staff that boosts attack and blood magic.

If Xebenkeck and the rage demons are defeated sufficiently far from the tome room (they can be kited back to the entrance of the cavern without issue) the party will drop out of combat. Again, however, the remaining guards in the tome room must be defeated before the achievement or trophy is granted.

Strategy
The battle with Xebenkeck can be made significantly easier by running back to the first room when combat commences; the reinforcements will still spawn in the room containing the grimoire allowing you to battle the first wave of foes in peace. This battle is made easier if you have a Hawke who is a Spirit Healer and the healing aura mode is turned on as Hawke and the companions will regenerate health, in most cases, before dying and if they do Hawke will be able to revive them when they fall. The more one invests in these skills the better they become.

Xebenkeck herself can also be safely pulled out of the room and fought by herself after the first wave of enemies are dealt with, so long as your party is sufficiently far enough when pulling the first wave. In this manner, a player can deal with the first wave, then Xebenkeck (who will summon a Revenant to her side), then the final wave using this strategy.

Note

 * According to the quoted codex page above, Xebenkeck has some sort of relationship with Gaxkang the Unbound along with two other demons. It may be possible that in future games the other two of the 'Forbidden Ones' (Imshael and The Formless One) could be major boss encounters as Xebenkeck and Gaxkang the Unbound are in the first two games. This leaves Imshael and the Formless One to be the be the remaining demons that must be defeated.
 * The codex page also leaves room to speculate, that those demons not only taught the Tevinter Magisters blood magic, but also might have incited them to worship the Old Gods instead of the Maker, taking revenge for abandoning His first children into the fade in favor of His second.

Trivia

 * The achievement "Exorcist" requires the player to find and kill Xebenkeck.