Demoniacally is the adverb of demoniacal: in a wild, uncontrolled, and evil way. Someone may grin demoniacally, or stare demoniacally — the manner feels driven, frightening, not quite human. Though actually, writers sometimes stretch it toward “with frightening energy,” as in working demoniacally through the night, where the “evil” part has thinned out. Like the adjective, it is rare and literary. In everyday English people more often say “like a demon,” “wildly,” or “in a demonic way.”