Yeah, she's supposed to be Grendel's Momma, and then she seduces Beowulf and has his half-demon babies, who further terrorizes ... whoever.
That's what I've heard, anyway. And they're trying to make a big deal about the nudity/sex scene somewhere in there, where a nekkid Jolie-computer model seduces Beowulf. But I don't know. It seems like it would be pretty easy to change, especially in a fully computer-generated movie.
-kv