As I remember it, Giles stated that Glorificus (Glory) was a Hell God, she ruled a world with two other Hell Gods. They feared her and caste her out, she was put on earth and contained in a human form, her prison. Glory (Glorificus) wanted the key to get home, she increased her personal power by 'brain-sucking' humans, when that power was low she needed a boost to maintain her power. That 'spazz attack' thing was just her low-level indicator at work.