GoW 2. The first, Title Update 6, adds a feature called "Social Matches" which allows individual players or parties to drop in or out of ongoing games (yep, even Horde mode matches) without interruption. But you knew that already, right?
The only catch is that you can only use social matching if you own all of the game's map packs which have luckily been slashed to $10 for the lot of them (that's the second prong, by the way). Individual prices have been cut as well. Other balancing and fixes implemented by Title Update 6 (a full list of which is available right here) will be available to all players.
To mark the occasion, players will get triple experience gain from May 27 to June 1, which we guess is technically a third prong. Yep, that's right: We've got a rare promotional trident on our hands.