Karthus, the Deathsinger

Whenever a player mills a card, put a +1/+1
counter on Karthus.
Whenever Karthus enters the battlefield or deals combat damage, target opponent mills a card.
If Karthus would die, regenerate him, then
exile him at your next upkeep.
If Karthus would leave the battlefield, exile him,
then cast him from exile without paying his mana
cost with suspend 4.