Beseech the Mirror At first glance, Beseech the Mirror might not seem all that special
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Foil/hologram cards and art cards mostly
Once on the board, Terra immediately gains flight (which makes her harder to block unless players come prepared with creatures that have reach) and can bring creatures back from the graveyard every time she deals damage
With so many good instants available, once a player untapped with a Shadowmage Infiltrator and an otherwise empty board the game was practically over, and if there was some small threat left over it could be dealt with in due time
Overencumbered, Rest in Peace, Stony Silence, Aura of Silence, and Back to Basics are just a few examples of powerful enchantments that interact with your opponents