The Badb is a goddess of fate and violence; it is she, along with her sisters Macha and Nemain, who determines the casualties of war. She is a warrior, a seer, and a carrion-eater. She is also, in an odd way, Arthur, or at least she will be.
The Badb is a composite being, an amalgam of many, many creatures, sentient and otherwise. They have all been absorbed into her purpose and her power; and since birth, Arthur has been slated to be so absorbed himself.
Arthur is at first unaware of this fate, but it is revealed to him as the game progresses. Whether he embraces it or flees from it is up to him.
However, once he realizes what and who he is, or will be, he also grows into certain powers and capabilities. He can talk with and control all corvids, familiars of Badb, and on entering a body of water, he can instantly re-emerge at other point in it, provided he moves with the tides or current. Further, he is given a small taste of the Badb's true nature: in battle, his hands are death, and simply a touch and his will can remove the life from any opponent.
But all these things come at a price, namely, Arthur's individuality. The more he takes from the Badb without, the more the Badb seeps within. If he wants any chance to fight the inevitable, he must use these gifts with the utmost stringency.
In-game effects:
The picture, badb.jpeg, above, is an illustration by Gustave Doré taken from Edgar Allen Poe's The Raven. So far as I am aware, it is in the Public Domain.