Late 70s early 80s movie. 2 kids one boy one girl get abducted by aliens. Years later a man becomes a serial killer for those aliens has a green glowing alien knife. Kills and cuts up the bodies and throws them into this cave hole that’s space/time vortex. The killer I think kidnaps the little girl, now a grown up and the boy, now a man, confronts the killer. The killer stabs the man with the knife and it doesn’t hurt him cause he was abducted and changed when he was little. I thought the killer was portrayed by Tracy Walter. Not sure. Thanks.