Edda of Burdens, book 2: By the Mountain Bound
Elizabeth Bear2009

For five hundred years the immortal Children of the Light, einherjar and valkyrie, have lived together in the North of Valdyrgard. They were born out of the Sea, each with a shining crystal sword in his or her hand; they are Angels of Light created in the formation of a new world. But three have come before them, from the death-throes of the old world, Midgard: the world-girdling Serpent, Bearer of Burdens; the Wolf Fenris, eater of the Sun, who now takes the form of an einherjar; and his demon sister, stealer of souls.

The Children spend their days feasting, fighting, hunting, and guarding their human charges. But one dreadful day a woman is washed up from the sea, a Lady who is no mortal, though she is not valkyrie either. Thus begins the breaking of the Children of the Light, the tarnishing of their power, and the death of Valdyrgard.

By the Mountain Bound is a prequel to Elizabeth Bear's All the Windwracked Stars.