Val'kyr

Val'kyr are a type of undead vrykul, and seem to be a recent addition to the armies of the Scourge and later the Forsaken. They act as the Lich King's agents in the spirit world, being described by Thoralius the Wise as being "fearsome creatures made of pure nightmare" that "now stand watch over the realm of the elements".

Background
The val'kyr are a creation of the Lich King. They were created from the female vrykul of Valkyrion, who fought and trained to receive the Lich King's unusual "gift." At least one example exists of a female vrykul being turned into a val'kyr at another location as well. Those that were deemed worthy underwent the transformation from simple vrykul to val'kyr. It is said that the val'kyr serve to judge the fate of vrykul combatants, transforming the favored contestants to ymirjar, and the failed contestants to the ranks of the undead vargul.

Beyond this, the Val'kyr Battle-maidens also watched the fates of the training death knights of Acherus, making sure the newly born warriors didn't fall in battle before their time. In fact, these val'kyr were acting much like the spirit healers of the undead Scourge. If they fell in battle, the val'kyr would return them to their unlife. During the Battle for the Ebon Hold shortly after the death knights are freed, they are level 60 elite enemies aiding Scourge remains.

The Val'kyr Annhylde the Caller is a part of the Ingvar the Plunderer encounter in Utgarde Keep. Upon Ingvar's death, Annhylde judged Ingvar and found him worthy. Annhylde resurrects him and begins phase two of the fight giving him a second chance. Fjola Lightbane and Eydis Darkbane are two very powerful Val'kyr that were originally sent by the Lich King along with their Vengeful Val'kyr army to disrupt, destroy, and terrorize the Argent Tournament and its citizens. But during this attack Argent Confessor Paletress comes to the defense of the tournament and battles the twins. Paletress is victorious, and Fjola and Eydis are captured. Their bonds are maintained by Priestess Alorah and Priest Grimmin. They later appear as the "final" challenge of the coliseum after the raid has defeated the Faction Champions.

Joining the Forsaken
With the Lich King's death, the val'kyr had no one to rule over them, to tell them what to do. They had nobody to serve. And so, the val'kyr turned to the one person who would have a use for their unique talents - Sylvanas Windrunner, Banshee Queen of the Forsaken. Sylvanas was left in a dilemma after the death of the Lich King as well - her unlife devoted to seeking out vengeance against the being that had created her. Once that vengeance was complete, Sylvanas was left with the unsettling decision of what to do next - both for herself, and for her people. The forsaken had one major flaw working against them; they could not reproduce. The Lich King was the one who created the Scourge, and now that the Lich King was gone, there were no Scourge left to turn to forsaken. The val'kyr were her salvation and solution to her dilemma. Since the val'kyr could raise the undead, they were essentially the key to keeping the forsaken "alive" as a viable race. With the val'kyr, new forsaken could continue to be created. And so Sylvanas made a pact with the val'kyr of Northrend, promising them something in return for what they did for her - continue to populate her motley race of living dead. The terms and agreements of that pact are an unknown at this point, but we do know this: as a result of the pact, the val'kyr will gladly give their lives to keep Sylvanas alive. The Val'kyr have been deployed into several battles since their conscription, notably throughout Silverpine and Andorhal, both of which were sound Forsaken victories due to the Val'kyr's input. As the Val'kyr are unable to reanimate worgen the surviving leaders and refugees of Southshore were willing to take the worgen curse and became Hillsbrad Worgen to prevent the Forsaken from raising them into undeath.

Inspiration
"Val'kyr" is an obvious reference to the Valkyries of Norse myth — minor female deities in the service of Odin. Their duty was to choose the most heroic spirits of slain warriors and take them to Valhalla, where they would serve at Odin's side in the Ragnarök.

Speculation
There are striking similarities between the Spirit Healers and the Val'kyr: Both val'kyr and spirit healers possess the ability to ressurect the dead. In the case of spirit healers, the dead are returned to life, in the case of the val'kyr, those raised are undead. Both val'kyr and spirit healers exist on the Spirit world, this much was shown in Wyrmskull Village. Both val'kyr and spirit healers possess wings and an ethereal glow, reminiscent of angels referenced in real life religion. Though both beings are similar in shape and in the actions they perform, they don't really have a connection with each other. Val'kyr are capable of moving between the spirit plane and the material plane - the spirit healers of Azeroth are either unable or unwilling to travel between the two. While both creatures can resurrect, there is a critical difference in how the mortals that are resurrected actually return to life.

See also: Know Your Lore: The mysterious connection between spirit healers and the Val'kyr

Without the Lich King's direct empowerment, the val'kyr's necromantic abilities seems to have been changed somewhat. For one, only human corpses are valid for reanimation for now and they are unable to reanimate the worgen. On the other hand it is possible that the Val'kyr under the Lich King were also unable to reanimate the worgen.