In Emperor (a six player format), two players are the emperors, and the remaining four players are the guards (two for each emperor). Players can only attack, target, and affect the players sitting immediately next to them. If an emperor dies, his or her team loses.
The main strategy for the emperor player is to use cards that give everybody bonuses (card draws, mana production, creature power, life gain, etc.) This enables the emperor's guards to play larger threats more quickly and ultimately should cause the opponents to lose due to being overwhelmed by powerful threats and answers.
Some Emperor formats ban alternate win cards (Barren Glory, Mortal Combat, etc.) to prevent abuse of the amount of time it would take to get past the emperor's guards and stop that condition from happening. Generally by suspending a Greater Gargadon with Barren Glory to sacrifice all of his other permanents or use Cephalid Illusionist and Shuko to empty one's deck into their graveyard with Mortal Combat.