Since we'll be leaning on Commander damage on a board after removal to kill, I have a few ways to make the Witch-King unblockable, including Shizo

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While not exactly a chase card from the set so far, itll wind up being a power player by this time next year
Westvale Abbey / Ormendahl, Profane Prince Westvale Abbey has a black color identity so it can't be played in every deck, but Ormendahl, Profane Prince is certainly worth going after in decks that can play it
Other Budget Upgrades To Consider Tricky Terrain is focused almost entirely on lands, but Omo's ability works just as well on creatures
It also has the potential to be the biggest eldrazi, caring about not just cards exiled with its own abilities, but from anywhere, making it paltry to get plenty of +1/+1 counters on it without casting