It begins with a single sapling, pushing through the earth as the days shorten and the nights grow darker. Perhaps growing in a town square, sometimes hidden in an overgrown garden. Most frequently, it will take root somewhere in the wilderness of the realms, unseen and unnoticed. The sapling is not much to look at save for one detail- even in heat of the sun its leaves are frozen solid.
Those who have heard the legend will act quickly. A mage or priest may dig the sapling up, set a fire where it stood and salt the earth around it. The fire will not be permitted to burn out until the first shoots of spring arrive. More nomadic races will simply up and leave, traveling without rest throughout the winter, always casting an eye into the shadows.
If ignored or unnoticed, within a few days the sapling will have grown into a small Witch Hazel tree. Its warped leaves hard and as cold as ice. Any attempt to remove the tree now will fail. However carefully it is dug out, however large the fire, within a few hours the Witch Hazel will have returned.
By the end of the first week, other trees will start to grow. Denizens of the mortal realms may wake to find a forest of Oak, Ash and Birch filling their camps or roads. Despite the cold and the changing season, most trees still bear their leaves, but every leaf is frozen solid. Survivors speak of attempts to clear a path with fire or axe, only to turn and find new trees have grown. They may, reluctantly, also speak of the sound- a crystalline ringing, faint at first but growing steadily louder each day.
By the end of the first month, the snow will set in, covering the trees and ground in a thick white shroud. By then the Herald walks amongst the trees, and it is too late to flee.
Those too close to the Witch Hazel will be swallowed by the forest, be it an Orruk camp, human settlement or even a city, it will not be seen again until the Spring, and the bodies of its inhabitants may not thaw until well into the summer. Those on the outskirts must stockpile food and prepare for siege, for every day throughout the winter the forest will draw closer. And every night, the Sylvaneth of the Witch Hazel grove will emerge from the snow, their limbs stiff and cold, their eyes and Lamentiri glowing blue, following the Herald into battle.