Gweyir, Rogue General

Shroud
Gweyir's power is equal to the total mana value
of creatures in opponents' graveyards. Gweyir's toughness is equal to that value plus 1.
On your upkeep, sacrifice a creature.
On your end step the player to your left gains
control of Gweyir.
When Gweyir leaves the battlefield, you lose the
game.