終わり無き地平線

If a different player gains control of Endless Horizons, its ability will trigger at the beginning of that player’s upkeep, but it won’t do anything when it resolves.

If Endless Horizons leaves the battlefield, the remaining Plains cards stay exiled. If Endless Horizons returns to the battlefield, it’s a new object with no relation to its previous existence. The previously exiled Plains cards will still stay exiled.