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It has flash, meaning you can hold up all your countermagic until the last second

Shout-out to Crawlspace, which was also reprinted in this set

Horrors dont have too much of a unified identity across Magic, though many of them deal with mill and cards in exile, a theme thats been strengthened by Commander precons like Mind Flayarrrs and sets like Duskmourn: House of Horror

What we can assume is that whoever added the X to designate the single card as a filler did not have access to the sheets source file where they would have been able to simply remove the individual card

Emerald Shimmering Skies has been pretty good for Emerald