Geological / environmental event
It is not quite known what set off the lava flows that created the Flowing Peak, but it hit both fey and trees hard enough that they came together to contain the catastrophe.
The first sign of the flowing peak was a forest fire. These were very coming due to the war between the intelligent trees and the fey. However, this forest fire did not stop after a few days like it normally would. This fire instead kept growing in all directions. While this did help the fey in their war efforts they also increasingly became worried about what would be left should they win by this forest fire. As such the fey began fighting the fires and were soon finding themselves working alongside the trees. As they increased their cooperation they were able to slowly fight back the fires until a giant mound of everflowing lava came into sight. This hill seemed to be entirely liquid itself but still able to produce evermore lava as the lava around it cooled down. As the two sides had already put their differences aside for this problem, they started digging trenches where water would flow freely due to magic and keep the lava contained. They eventually succeeded at this, however, during their efforts the mound grew ever taller and now stands as a sole mountain called the Flowring Peak.