Wrexial, the Risen Deep Wrexial, the Risen Deep is a commander that rewards milling your foes by letting you cast an instant or sorcery from their graveyard for free when Wrexial hits them
Interaction versus removal has never been better, and though March of Swirling Mists is likely to be better for most decks, Slip Out the Back definitely has a place, even outside of +1/+1 counters decks
Traditionally there have been large amounts of cards that change hands at various conventions across the world (most notably the Star City Games Open events and MagicFests), but these have been shuttered due to COVID restrictions
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Where most Unknown Event cards employ puns, or make fun of their reference targets in some way, Shen plays it very straight