I’m pretty sure he just performed the standard demon summoning ritual where he gains something in exchange for his soul.

[Ray looks over at Venkman. Yeah, he’s really starting to notice how weird Egon is being too, and while he’s not fueled by being a jerk, he’s still really curious about what’s going on.]

You alright, Spengs? The way you’ve been reacting whenever we bring up the slime really makes it sound like something happened during the experiment.