GREGOR: What do you know of the beast?
BUFFY: Strong. Fast. Hellgod.
GREGOR: From a dimension of unspeakable torment.
BUFFY: A demon dimension. I know. She ruled with two other hellgods, right?
GREGOR: Along with the beast they were a triumvirate of suffering and despair. Ruling with equal vengeance. But the beast's power grew beyond even what they could conceive. As did her lust for pain and misery. They looked upon her, what she had become ... and trembled.
BUFFY: (nervously) A god afraid?
GREGOR: Such was her power. They feared she would attempt to seize their dimension for herself, and decided to strike first. A great battle erupted. In the end, they stood victorious over the beast ... barely. She was cast out. Banished to this lower plane of existence, forced to live and eventually die trapped within the body of a mortal ... a newborn male, created as her prison. That is the beast's ... only weakness.
BUFFY: Kill the man ... and the god dies.
They were too weak too kill her.