Much hyped at the time, Shiny Entertainment’s 2000 release Messiah is available today through Steam. Oddly, despite their recent financial perils and attempted IP auction, Interplay are still listed as the publisher.
There’s no indication that this version is any different from the one released through GOG in 2008, and the GOG version remains cheaper ($10, versus $15 on Steam once the early promotion ends). So unless any kind of compatibility improvements have been made for the Steam release, you’re almost certainly better off sticking with GOG.
In Messiah, you play as angelic cherub Bob; God’s emissary on Earth (or a cyberpunk version of Earth, anyway). Your task is to clean up the sinners, as a sort of one-baby alternative to a massive flood (God’s usual method of cleaning house). To that end, you’re able to possess “any person, animal or genetically engineered being” and go on a spiritually sanctioned murder spree.