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.
It's one of the greatest local multiplayer experiences I've had. It's one of those games that is incredibly frustrating in a good way.
Not sure about playing it alone, but with friends it is horrifyingly fun and really fleshed out.
Presentation is incredible, and there's a lot to unlock. I'm addicted.