So, we know the usual EDH free-for-all can get a little stale after a while. Why not mix it up with cards that can grant you special objectives or open up more diplomatic drives?
Based off the Servant classes in Fate/Stay Night, the Identities are cards you don't cast - they are in your command zone, face-down unless stated otherwise, and grant you a secret objective to accomplish. Some involve merely surviving. Others force you to seek out and kill players with your own hands.
Before the game begins, players are dealt their identities at random. There will always be a Ruler, Saber, and an Assassin. All other identities are included at random. Everyone will reveal their identity when the game ends for any reason, like someone achieving a card's win condition or accomplishing their identity's objective. This is how identities like Lancer and Rider win - by having the game end without them dying. It is possible for multiple players to win the game at the same time.
The identities merely add alternative objectives you can achieve. You can still win the game the usual way by eliminating everyone else.