Valkyries (from old Norse valkyrja, "Chooser of the Slain") are the servants of the gods Odin and Freyja. They are essentially angels of death, hosts of female figures who decide which soldiers die in battle and which live.
In Rites of Passage, Ragnar sees a vision of the Valkyries carrying fallen warriors off to Valhalla, appearing along with Odin and a flock of ravens. They are refered to at various points in the show.
While half of those who die in battle go to the goddess Freyja's afterlife field Fólkvangr, the other half are selected and brought by the valkyries to the afterlife hall of the slain, Valhalla, ruled over by the god Odin. There, the deceased warriors become einherjar. When the einherjar are not preparing for the events of Ragnarök, the valkyries bear them mead. Valkyries also appear as lovers of heroes and other mortals, where they are sometimes described as the daughters of royalty, sometimes accompanied by ravens, and sometimes connected to swans or horses.
Some have theorized that the Valkyries are not individuals, but where considered to be a collective of aspects of a war goddess. Scholar Hilda Ellis Davidson has stated from "early times" the Germanic peoples "believed in fierce female spirits doing the command of the war god, stirring up disorder, taking part in battle, seizing and perhaps devouring the slain". The Valkyries are frequently associated with birds in Norse art, maybe due to the various birds coming to consume the flesh of those killed in battle.
Although modern depictions often show the Valkyries as angel-like beings taking fallen warriors to Valhalla but the Norse depictions are darker and more sinister. Odin sends off the Valkyries to give victory to those favoured by him. The Valkyries would change the direction of arrows, causing sheilds and sword to break and warriors to slip causing their demise. After the battle was over, the Valkyries would choose the bravest and most skilled among the fallen and carry them to Valhalla and Folkvangr.