In Curse of Strahd, the PCs go to a place where they are Dark Gifts, powers, from sinister forces. Each one of these comes with a price, though that's not spelled out to the characters before the gifts are accepted. One of the side effects includes the PCs eyes melting out of their sockets (they can see normally though), losing all their teeth, being terrified of fire, their bodies become covered in oily black fur, etc., etc. So I guess there's precendence for this kind of thing in an AD&D adventure.
While I wouldn't worry too much about a Call of Cthulhu game, I would worry about this in a D&D game. Not because I'm worried anyone is going to be traumatized or harmed by body horror, but because it's not what we typically expect in a game like D&D. I'm not a big fan of the "required" part, but I'd certainly talk to my players about the possibility of this kind of thing happening to make sure they were onboard.