The Valkyries are warrior women who serve Odin, the chief of all the Asgardian gods. It is they who decide which of those who die on battle are worthy to be taken to Valhalla. These fallen heroes then begin training to fight during Ragnarok.
The Valkyries in the Xenaverse, however, are portrayed as acting as more of messengers between the Midgard and Valhalla, serving Odin faithfully. The majority of them have immortal blood running through their veins and have special powers. Women who are valiant warriors become Valkyries and are able to ride the sacred Sky Horses of the Valkyries. Odin teaches his favorite Valkyries special runes or magic.