Unhallowed Autumn

At the beginning of your end step, put a -1/-1
counter on target creature. Then, if there are 3 or
more -1/-1 counters among creatures on the battlefield, sacrifice Unhallowed Autumn, then
you may search your library or graveyard for a
card named Winter's Grip, reveal it, and put it
onto the battlefield. If you searched your library
this way, shuffle it.
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When leaves wither and brown, dark forces emerge.