What would dryads be like, a thousand years after such an event, still connected to their dead-but-preserved trees? Insane and angry, yes, but what else? Would there be physical changes? How would their abilities be different? Would they be some form of undead, ghosts or something else? Is there such a thing as undead fey?
That's... a pretty cool scenario.
I don't think dryads necessarily die when their tree is destroyed, at least not in this edition's lore. But lets see...
- The forest that was destroyed may have an "echo" somewhere, let's say in the shadowfell, that survived, and the dryads (or some of them) may have migrated there to escape destruction. Now they are coming back as the forest regenerates. Maybe the shadowfell forest echoes back into the material world, or after a thousand years, the new forest is starting to generade its own echo, superseding that of the old forest and allowing the dryads to come back. Evil shadowfell dryads sounds fun, and so is a region of overlapping shadowfell wilderness (or whatever).
- As said earlier, so many dryads losing their trees and going mad simultaneously may be enough of a traumatic event for them to become ghosts - the tortured ghosts of mad dryads, filling a whole region!
- I don't think a thousand years is enough time for trees and plants to petrify, but lets says it is (because fantasy). With erosion, these petrified trees are starting to surface, and along them, petrified dryads. From the concept of "petrified" dryads, you can go the earth elemental route, or even the undead route (although not necessarily evil), or literally, statuesque stone dryads in positions of agony waiting to be reverted to life.
- Perhaps it isn't about the region anymore and the mad dryads spread to the world finding new way of sustaining their existence. Vampire dryads sounds like a cool monster to introduce in a campaign.
- Or after decades of madness, the dryads recovered as nature regained its rights, and since then, there has been an ongoing war between the dryads and the fire elementals (or salamanders or whatever) of the volcano. Good opportunity to introduce a violent, warrior-culture dryads contrasting with their usual gentle nature.
- Or the dryads spread to the world and eventually regained their sanity through contact with mortals. Introduce "free dryads" as a PC race!