The typical Eternal Forest should include at least one of these features:
Usually, the Eternal Forest occupies most of the map of the known lands: all who travel the continent have to cross it or sail its rivers. The Player Characters are likely to spend a lot of time in the shadow of its trees, exploring many different areas and stumbling upon dangers and discoveries that will change and evolve the forest over the course of the campaign. Who knows… maybe one day they might even reach those secrets hidden far below its roots!
The sheer size of the Eternal Forest makes it an ideal hideout for a Villain, who might also take advantage of it and ambush the heroes. Also, some antagonists will be interested in the treasures and the miraculous machines hidden in the depths of the forest – some out of greed, and some hoping to wield their powers for good, unaware of their collateral effects.