Once the Royal Thorns have at least 3 stacks of thorns, and at least one of them is thorn-hunters, they can use their minions to wall themselves in against the side of the map. The ancient vampires can try to maneuver themselves to lengthen the royal thorn's path to the wall, but starting with turn 4 the royal thorn can use summoned thorns to hold a path open and the ancient vampire can no longer stall; even before that the royal thorn can advance one space by tacking around the ancient vampire on turns 2 and 3.
So it's actually less likely than at first glance - the AV must get 17 critical hits in 19 rounds, at a 17% chance of a critical per attack; the royal thorn must summon no more than 21 thorns in any given germinate, AND those thorns must always be thorn warriors, not thorn hunters (~8% chance each). Giving total odds of success of roughly 3 in 1,000,000,000,000,000,000.
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