The Servant Emperors are the four most powerful servants in the world.
Dozer leads the fanatical, straitlaced and deadly efficient Dozer Abolish faction. He also has a country named after him, which he controls.
Morgunov leads the violent and unpredictable Morgunov Abolish faction. He is brilliant and insane.
Sermung leads the Vanguard, and is strong enough to take on both Morgunov and Dozer together and hold his own. He is the most powerful single being on Eleg, with the possible exception of Rasalased. Despite his brilliance both in combat and as a tactician, there is a growing movement that thinks him unfit to lead the Vanguard.
Sai-hee leads the Independents, who ferry souls, protect their own land and enforce the Old Law. She is the reason why neutral Reapers are safe to collect the souls of the dead without risk of being attacked or press-ganged by either side. She seems to be the weakest of the four Emperors, but could likely tip the balance in favor of either Abolish or Vanguard if either side offended her sufficiently to break neutrality.