The Morrígan was the Celtic goddess of war, life and death. A shape-shifter, she could appear in many different forms including a young woman, a crone, a child, or as a trio of sisters called Bodh, Macha and The Morrígan. As a war goddess she would appear in the form of a crow to warriors in battle and egg them on to great feats. It didn't really matter what side they were on; it would seem that she just liked a good fight.
A member of the magical race the Tuatha Dé Danann, she is part of the Ulster cycle of Irish legends and was a pretty badass lady. Or possibly still is...