Valnus, Larceny of the Polis

When Valnus, Larceny of the Polis enters the battlefield, choose a player.
As long as you control Valnus, the chosen player's graveyard is under your control. (That graveyard is no longer counted for effects and cost of the original owner's spells, but they do count for your spells and effects. Any new cards that are placed in that graveyard are also under your control.)