Darkness is Light Enough

The duty of the Valkyries of Valhalla is to safeguard the lives of the nine realms. They are to never mind the individual when there are many to think of. But one young Valkyrie attaches herself exclusively to the dark prince of Asgard. Is there a return from the darkness they both fall into? Or is the darkness light enough for them? Starts pre-Thor and continues into the Avengers.