Possessed by the Night

Possessed by the Night is a 1994 erotic thriller directed by Fred Olen Ray, who also cowrote and had a brief appearance in the film. It also stars Shannon Tweed.

What Howard and Peggy dont know is that Carol is conspiring with her partner Murray, Howards greedy agent, to steal his manuscript. Carol soon becomes possessed by the thing in the jar as well, playing sexual violent games with Howard and Peggy, terrorizing Peggy at every turn, and ultimately turning on Murray, leading to a hyperviolent climax which has nearly everyone blasting away at their coconspirator with automatic weaponry.

Source: Wikipedia