In Norse mythology, the Valkyries are supernatural beings that guide half of slain warriors to the afterlife in Valhalla ruled by the god Odin (the other half go to the meadow Fólkvangr, ruled by the goddess Freya), where the Valkyries give them mead. Valkyries are sometimes the lovers of heroes and men. They are often shown with ravens, swans, or horses.
Here a Valkyrie rides a winged horse over a large city, presumably London, although Hughes has added mountains in the distance to make for a more dramatic landscape. Valkyries were magically able to ride through the air. This Valkyrie is probably Mist, she whose name means ‘cloud.’