The zomok is a rejuvenating but dangerous entity of nature—think the Forest Spirit from Princess Mononoke in vaguely draconic form. It is the true master of the forest—actually, it is the forest; Pathfinder #36: Sound of a Thousand Screams describes a zomok passing through the forest like a wave, the trees themselves bending to become part of the its form and then returning to their places. Zomoks have the spell-like abilities to create shambling mounds, remake oaks into treants, command plant creatures, and otherwise turn the very woods into a weapon. And let’s not forget those nasty breath weapon, swallow hole, and trample attacks.
To really get a feel for the zomok, the original entry from Pathfinder #36 really is the way to go—it describes how zomoks are playful in springtime and dangerous and hungry in winter, how they ally with dryads and fey, and how they even eat shambling mounds so that their swallowed victims have something to fight inside their stomachs! Oh, and they can heal 180 points of damage just by forest stepping. So good luck trying to kill one on the first try.
In a campaign that’s stopping short of Level 14 or so, the zomok might be the final boss of the entire campaign. In a campaign going all the way to 20, the zomok is the complicating force, the one obstacle the PCs didn't need that demands attention at the worst possible moment (Harry Dresden fans know what I’m talking about). Nature has its own laws and demands its own particular forms of retribution—and it will always be the zomok who determines that timetable, not the PCs.
Cold riders and winter hags band together in an alliance to banish summer entirely and bring a winterwight into the world. They succeed, turning spring into fall and kicking off a new ice age. The unexpected corollary to this is that zomoks across the hemisphere go mad as their forests starve. Denied the warm rays of the sun, the zomoks go on feeding frenzies, devouring animals, people, and even whole towns in order to return the energy to their home forests.
The dryad queen is not bound in one place like her sister-subjects, for her home tree became the spine of a zomok long ago. Able to ride through the Great Meredi Forest at will, she has gone from being a voice for dryads to being a cruel faerie despot. After the zomok smashes the elven city of Quistarthi into flinders, the Speaker of the Candles decides that the dryad queen and her plant steed must be stopped.
When a zomok turns bad, the whole forest turns bad. The Larchmarrow turns twisted and dark after the poisoning of its zomok, who now leads an army blighted fey satyrs (see the Inner Sea Bestiary) on guerrilla raids. The culprit is an ancient forest dragon who sees the opportunity to remake the borders of his domain, smash his enemies at no risk to himself, and cause the miscarriage of the jinushigami egg that he suspects is buried in the zomok’s core.
—Pathfinder #36 88–89 & Pathfinder Bestiary 4 287