Dark daeva. It is filled with an insatiable craving to see the living suffer.
Nightmare is an enemy from Vagrant Story. It is fought at the Grand Cathedral's room What Ails You, Kills You.
When Ashley enters the room, a curious disc flies around him, finally stopping midair. It has a half-face of the sun and the moon, and the Nightmare is spawned from it.
As there is no spell to increase Light affinity you will have to rely on casting "Degenerate" on The Nightmare, followed by "Herakles" and "Prostasia" on yourself. Equip a Blunt weapon and attack it's body.