Lonely deterioration

Geological / environmental event

The entire forest, including the parts taken by the Northern Inferno, grow impassable due to Arboreas broken mind.

A forest might regrow quickly under dryad, but not if the dryad is in serious pain. After the Northern Inferno took a large part of the forest, pain is inevitable. Even as the forest began to grow again, dulling that pain, the pain of loosing her druid friends took hold of her.