Normally I like to point out in-game reasons why monsters are awesome, but with moss trolls it’s all about the stat block: They’re CR 3, AC 15 (flat-footed 11), and shaken by fire. This means that they’re perfect (tough but not lethal) boss monsters for a 2nd or 3rd-level party (or even 1st-level one, if PCs are smart and plan ahead).
Even this delightful bit of flavor text—“When a moss troll spots prey, it reaches or leaps down from above, heedless of dropping great distances in its eagerness to sate its ravenous appetite” (emphasis mine)—gives you a way to make moss trolls seem badass and yet still stack the deck in a young party’s favor.
Picture it: The moss troll drops to attack. It lands with a thud, just missing the PCs. You describe how the monster shrugs off its bad landing, its regeneration relocating the troll’s twisted knee back into place right before their very eyes…but secretly only you know that the moss troll’s fall has done enough damage that the PCs have a round or two to get their act together.
And that’s great! Low-level adventures are super hard to end satisfactorily with a fight. A moss troll is a real monster, a real threat, a real challenge to overcome with both brains and brawn—but with plenty of Achilles heels built in.
And once your PCs are more experienced? Throw a troop of 4–8 at them right after they’ve shelled out for vials of acid, and watch their dismay as their gold pieces go down the drain. Or set them up for a jungle encounter with gorillas and dire apes—perhaps a specimen-collecting exhibition—only to have moss trolls come brachiating through the trees instead.
Oh yeah, the fact they can turn into a tree is pretty cool, too. Add some levels of rogue or barbarian to that and you’ve got a pants-wettingly stealthy and fast ambush. Have fun!
Fresh from its mother’s nest, a very young black dragon strikes out on its own. Quickly growing used to throwing its weight around, it suffers its first setback when a gang of moss trolls shrugs off its acid spit and regenerates its talon slashes. Now the dragon licks its wounds and plots its next move, as do the mossy humanoids. Both the dragon and the trolls will rally to defend the swamp from outsiders, but a canny party might be able to set the two sides against each other again.
The Burrowback Boys have lamented their lost brother for a year. Now they have discovered he is alive—captured by a traveling circus during its annual rounds and kept bound by his fear of fire. Using a muck-encrusted but serviceable trebuchet they found moldering in a clearing, they plan to fling giant leeches into town as a distraction, free their brother, and split the corpse of the circus’s sorcerer master of ceremonies among them.
Born from a tree poisoned by hag blight, the dryad Merifex has always been a bit wrong. Just how wrong became apparent when she tried to tree meld with a disguised moss troll—and promptly fell in love with the brutish creature. Now she lures men to feed her monstrous lover. Assuming adventurers don’t stumble upon Merifex on their own, the satyr Cheridwen will beg for their assistance “rescuing” her. In truth he is madly jealous of the troll—Merifex spurned him years ago—and he intends to rob whoever comes out the winner.
—Pathfinder Bestiary 3 273
“What the deuce?!” you all exclaim, monocles cascading from your faces into your Martinis as you interrobang in shock. “You neglected the mosquito swarm, you cad!” Ah, but rest easy, it is here.