Crawl is a beautiful combination of teamwork, skill, and luck.
In Crawl, your job is to venture through various rooms, where you try to find monsters, which are spawned by other players. Upon slaying these monsters, you gain experience points, which you can use to fight the boss after you have collected enough.
If your friend (a monster) kills you, you swap places, meaning you spawn the monsters, and they fight them. When you kill the adventurer, you gain "hero blood" which you can trade for gold to spend at a shop, spending it on weapons, allies, and abilities.
When you eventually fight the boss, the game doesn't stop being multiplayer. Oh, no. The non-adventurers must actually possess parts of the massive boss and work together to defeat the adventurer. If the adventurer kills the boss, he/she wins. If the boss "kills" (you teleport back aways) the adventurer 3 times, the game is over.
When a game is over, you can unlock new monsters which players can transform their previous monsters into. You also unlock new abilities for monsters and adventurers, etcetera.
Overall, I think that Crawl is a masterpiece. The pixel-art blends very well with the shifts in scene, the soudtrack is amazing, and the gameplay itself is one-of-a-kind.
I rate this game 10/10, and it's still in development. I highly suggest you buy this game if you're even remotely interested.