The theme of grotesque, nightmarish video game enemies having once been human is something that developers seem to favour more and more in recent years. Somehow, knowing that these creatures were just like us at an earlier point in their existence has a way of hitting home for the player - both as a warning and, in some cases, a grim foreshadowing.
Many video game premises revolve around planes of reality that mankind can only guess at: Heaven, Hell, outer space, and even the purgatorial space between spaces. These realms are not meant for mortal men, so is it really surprising that any amount of contact with these places can turn you and your foes into something else entirely?
The unknown is perhaps the greatest source of inspiration for storytelling, from fantasy and sci-fi to the likes of Lovecraftian Horror. With endless possibilities comes endless dangers, and many who poke their noses where they shouldn't end up losing their humanity along the way.
And that's not good news for your character who has to deal with them.