Treasure Chest

When Treasure Chest enters the battlefield, for each opponent, gain control of an artifact that player controls.
Whenever an artifact enters the battlefield under an opponent's control, you may pay {x}, where X is the number of artifacts you control and do not own. If you do, gain control of that artifact.
When Treasure Chest leaves the battlefield, return all artifacts you control and don't own to their owner's control and you lose 1 life for each artifact returned this way.