One of the borders of my map is a forest that was burned so badly and for such a great distance that it is currently impassable for humans. There is a once-great city on the edge of the burned forest that was damaged and is being rebuilt.
The city is home to both humans and centaurs who haven't interacted greatly in the past, but are on good enough terms that they are working together to rebuild the city and restore the forest. The Centaurs want it cleaned and restored, because it is where they lived. The Humans want it restored, because it was the major source of lumber for their nation.
It is a pre-gunpowder era with cannons and flint just being developed/ coming into use. Humans are not populous enough to destroy the forest through logging and so the logging causes no strife with the Centaurs. The Centaurs are living with the Humans because they can no longer live in the forest and have also begun to work with the humans outside the city to protect their nation.
What is beyond the forest is unknown because humans have never successfully traversed it before the burning and the centaurs never had a reason to leave it before.
What I need is an event that would burn the forest so thoroughly that it would force the Centaurs out, make it impassable and spark the cooperation between the Centaurs and humans. I thought of potentially a meteor or some other form of heavenly fire, but something like a lightning strike seems like a little bit of a cop out.