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Flying
When Boreas Charger leaves the battlefield, choose an opponent who controls more lands than you. Search your library for a number of Plains cards equal to the difference, reveal those cards, put one of them onto the battlefield tapped and the rest into your hand, then shuffle.
If no opponent controls more lands than you, you'll search your library and shuffle it, but you won't be able to find any Plains cards.
You can find fewer Plains cards than the difference, perhaps because you don't have that many Plains cards left in your library.
