A mancubus attacks you i think. he killed me first time i tried it but I fried him with a plasma gun. Then god knows what to do next. I'm stuck at the Cyberdemon level. I pump shot after shot into him and he won't bloomin' die. Damn my shotgun.
To kill the cyberdemon you have to use ur Soul Cube.
after 4 or 5 times of activating the soul cube it should die