A Dungeon-Crawler Where Your Friends Control the Monsters. Something like a Retro-Arcade PvP Bullet-Hell Dungeon-Crawling Roguelike-lite-alike?
I think Crawl is a fantastic game. It's one of those games where the mechanics are extremely simple (think Binding of Isaac or Minecraft) but still complex and deep due to the variety of items and monsters and things like that. It's super easy to pick up and quite fun to play, all only in the limited early access state that it is. And it has the mark of the truly excellent 'multiplayer-only' game that it is: it's even fun to play on your own with the bots.