Currently Karthus and Sion aren't considered defeated until their respective passives wear off. Which means that, for example, if Karthus kills his opponent _after_ having died, he wins the fight.
Which is pretty unfair thanks to the health burn that comes with not dealing damage to enemy champions that Prize fight and URF deathmatch feature: you need to win the fight with enough health to withstand the undead's blows AND the burn. If anything, removing the burn while Karthus is sole survivor of his gauntlet and in his passive form would ease the pain. Sion's passive looks fairer to me.
That's all folks. See you in the Blitz.