By the Mountain Bound — Elizabeth Bear

For five hundred years the immortal children of the Light have lived together in the north of Valdyrgard. Einherjar and valkyrie, they were born out of the sea, made of starlight, each with a shining crystal sword. 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 of the Light 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 no valkyrie either.

And then begins the breaking of children of the Light, and the tarnishing of their power, and the death of Valdyrgard.