The Shadow Druids come instantly to mind, but for your situation it sounds like you want an Anti-Druid. In the same manner that fallen paladins appeal to the evil powers and they take glee from granting them new and terrifying abilities (all the better for evil-ing) traditional powers interested in undeath, corruption and blight would be thrilled to count fallen druids as their thralls.Say, for instance, you have your average happy hippie tree hugging druid, and their friends, family and colleagues all get killed by the kingdom who is clear cutting a forest (for the greater good, of course, we need fleets to fight tyranny), they might snap. All of a sudden the natural balance seems like BS, and it's vengeance time for civilization. That's a dark place, mentally, so when oak-daddy, patron of the unwashed, stops granting spells and all of a sudden the god of pestilence and destruction starts whispering in their ears, boom, that's one fallen druid.
Add in some warped worldview, predator and prey mentality, cycle of life and death stuff, what have you. But make them good and crazy. "Humans have upset the balance with civilization, my army of undead squirrels are just going to set it right!" Plague carrier corpse creatures descend, and all of a sudden it's black death 2.0, now with an evil druid coven assassinating the clergy able to cast remove disease using their troupes of awakened racoon assassins.
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