If the Royal Thorn sows, at a minimum he creates 18 thorn-hunters (or 108 hitpoints in thorns, at defense 1). If the vampire vampire broke off to try and deal with the thorn-hunters, AND got a critical hit every single turn, he'd do about 30 damage a turn to them. So if the vampire spends time attacking the summoned thorns, every 3 turns he does 90 damage to summoned thorns at most, and at least 108 hitpoints of new thorns are created. He'll never catch up, so his only hope is to kill the royal thorn before it can summon too many supporting thorns. Even if he critical-hits the royal thorn every single turn, it's still only about 14 damage a turn, meaning he needs 26 consecutive critical hits (i.e., 26 turns) to kill the royal thorn.
But by turn 8 there can be 3 supporting thorn stacks in play, which is enough to blockade the royal thorn in a corner. Now the vampire has no choice but to attack summoned thorns, but the royal thorn can take all the time in the world and gradually summon more and more. Heck, the royal thorn can fill up the entire battle grid with thorns if he wants to, then they can all launch one massive salvo at the vampire.
In fact, with ideal luck the royal thorn can take down a troll at night. With any sort 1-space chokepoint (uncommon, but not unheard of on battlefields) the royal thorns can seal themselves away from the troll with just one germinate, before the trolls close to melee. With consistent ideal luck, the royal thorns could summon 37 9-hp thorn warriors every 3 turns (or 333hp of defense-3 blockers). With more consistent ideal luck, the troll would do only 107 damage per turn, or 321 damage every 3 turns - or it would take 4 turns to kill off what the royal thorn can create in 3. So given time the royal thorn could gradually create a whole series of thorns, then could slowly manipulate them around the map until the troll is surrounded by thorn warriors, and the rest of the map is filled with thorn hunters. That's about 50 thorn-hunter units of 37, or 1,850 thorn-hunters all hitting the troll at once. If a thorn-hunter gets a critical hit, it does 1.78 damage, so 1,850 of them hitting does close to 3,000 damage. So while the odds are so astronomical that you'd probably have to invent a whole new notation just to write them, AND it requires a favorable map (either a particularly large one, like in Ultrax, or one with a convenient 1-space chokepoint), I believe the royal thorn may be the only unit even theoretically capable of killing a troll at night. And it certainly can kill a vampire.
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