I had thoughts on evil druids, and it caused me to take a new look at trees.
Consider just how powerful an animated tree would be. It must hold up it's own immense weight just as a normal tree, and withstand windstorms which generate truly colossal forces against it.
Now, it is walking, talking, flexing it's branches ... and anything in it's way is history.
Now, when the entire forest gets involved, everything in their way - including assorted cities, countries, archwizards, super clerics, and even those industrially minded gnomes

are history.
So we have an evil druid. And the evil druid says:
For countless millennia, elves and men have chopped down the trees.
Now, it is time for the trees to chop down the men and elves.
Arise forest, and march in your countless billions to Green Glory!
Are you sure you want evil druids in your world?
(Paraphrased from The Illearth War, by Stephen Donaldson: Against the might of the Deep, even Fleshharrower's immense army was insignificant, a paltry insult hurled against an ocean. The trees overwhelmed the cunning of the ur-viles, the brute strength of the cavewights, and the mad, cornered fury of the myriad stonewarped creatures. Knife-wielders were slain, fires were stamped out, lore and force were overwhelmed. Then the trees obliterated all traces of their victims, in a storm of satisfied vengeance.)