Polygod is a randomly-generated roguelike FPS about a Faceless One, a legendary assassin with one arm who must prove himself to the seven deities by fighting through their arenas and defeating their Holy Champions in order to ascend to godhood. It features a colorful, minimalist visual style and a "brutal difficulty curve," and after nearly two years on Steam Early Access it will go into full release tomorrow.
The Faceless One also only has one gun, but he can improve it (and himself) in numerous interesting ways with Blessings, more than 100 character and weapon upgrades that can be purchased with the souls stolen from your fallen enemies. Co-op and competitive multiplayer modes are supported, and levels (including the Blessings) are "seeded," so they can be shared with other players for leaderboard competition.
And it is as difficult as billed: I played the Early Access release last year and it kicked my ass pretty hard. Even so, I enjoyed it quite a bit: It's very much a "pure" shooter, reminiscent of Lovely Planet but much busier and less insta-death, and there's a dark wit about it that I get a kick out of. The world is bright and cheery, but the specter of death hangs over everything—and nobody seems to mind.
The move to full release will probably involve an update of some sort, but in these days of Early Access it's as much a matter of semantics as anything else—which is to say, you can pick it up now if you like for $15, and the price will remain the same once Early Access is over.