In Norse Mythology...
A valkyrie, (in Old Norse, valkyrja, means literally, "chooser of the slain.") is a female figure who chooses those who may die in battle and those who may live. Selecting among half of those who die in battle (the other half go to the goddess Freyja's afterlife field Fólkvangr), the valkyries bring their chosen to the afterlife hall of the slain, Valhalla, ruled over by the god Odin. There, the deceased warriors become einherjar (Old Norse "single fighters") and train for the final battle of Ragnarok.
The Valkyries carried out the will of Odin in determining the victors in battle and of course the war. As each Valkyrie performed differing tasks according to Odin’s instruction, it was their prime duty to ride into the battlefield and choose the fallen heroes of the field. To be chosen by a Valkyrie and carried off on her white steed to Valhalla was considered an honor to the dying Viking warrior, for Valkyries only chose the bravest of the slain, gathering souls found deserving of an afterlife. They traveled far-and-wide searching for the dead in battlefields, oceans and seas for mortal men worthy of the grand hall. However, if the Viking warriors are deemed unworthy by the Valkyries, the goddess Hel in a cheerless underground world received them after their death.
The Valkyries were depicted as young, beautiful, but fierce women who dressed splendidly in full armor and swords when riding their horses. A horse created from air, and when they traveled to Earth, frost and dewdrops would fall from their manes onto the ground. The Valkyries were also Odin's messengers and when they ride forth on their errands, their armor causes the strange flickering lights known as the “Aurora Borealis” or Northern Lights.
In the world of the Runes Series, dying men warriors of purest soul are found worthy and offered a chance to become Valkyries, reapers of souls...
External References:
https://en.wikipedia.org/wiki/Valkyrie
http://www.hillcountryroofing.net/VQ/myths.htm
