Große Liebe (1966)

A young painter loses his lover and rich patron in a mysterious car accident. He searches the scene for clues, finds a gateway and disappears into it before his friend's eyes. What awaits him is a kind of otherworld where, in addition to a host of macabre spirits, his former lover appears. Like all this place's other occupants, she too attempts to eat the only obviously living being around.

Unfortunately the movie Große Liebe is not yet available on HBO Max.