Bang couldn’t describe it.
Explosion just about covered it, but failed due to it’s lack of onomatopaeic stucture.
Kaboom was just too comic, and just “Boom” couldn’t really be taken seriously.
There probably was no word in the language that could describe the explosion he heard, because it would loose the connatations of the stench of burning flesh mixed with the howls of pain from the not-quite-yet deceased. Because the explosion wasn’t the end of it, Even an explosion this big could not be the end of it, and it’s location - deep within thouands of acres of forest - was not conducive to the event ending quickly. Which is to say that when the superheated spell made it’s explosive impact, the results were death and fire, and the fire begat more death. Doing this in the summer season was not helping, as the dryness of the river did not