You are in the Ubellian Fire Pits, where you disturbed the hibernation of the mighty Red Dragon of Fire, Azymor. Try to avoid the moving pools of lava that Azymor controls, although it is impossible not to get burned, because most of Azymor’s abilities can either move the unstable soil, move the heroes around, or even target multiple heroes at the same time.
Putting the fire out at its source, as quickly as possible, is the best way to stop the pain. Kill Azymor to win this Boss Fight.