Doom RPG is an oddball entry in id Software's Doom series. It was originally released in 2005 for the mobile devices of the day—the pre-iPhone era, remember—and instead of being a straight-up shooter it incorporated a number of basic RPG elements. It did pretty well for itself too, earning high review scores and several mobile game awards, and spawning a sequel, entitled (of course) Doom 2 RPG.
It took 17 years for Doom RPG to finally come to PC by way of an unofficial fan port, but fortunately the wait for the sequel hasn't been quite as long—you can pick it up right now courtesy of Gec.Inc, the team that ported the original game.