Oshikiri (2000)

Oshikiri is a loner who has bad memories of his childhood and his grandfather, a brilliant scientist. He returns to his grandfather's mansion to try to remember what really happened to him. He finds a portal to a parallel universe, which his grandfather had originally discovered. From this portal comes his doppelganger dressed in a cloak and gas mask, who kills his fellow students with an ancient axe.

Unfortunately the movie Oshikiri is not yet available on HBO Max.