Fire. OH yeah.
The invention of black power in the 800s AD brought muskets, pistols, cannons, and more.
Big picture: The immortals have ceded Terran space to humans. These may indeed be the race to eventually succeed them. They truncated the space (making magic problematic), leaving Man to his Tech and hubris. In a trillion years, maybe Tech WILL supplant magic; wait and see.
But learning from the experience, they discussed matters with the plane of Fire, and decided to curtail this effect in promising magic-based civilizations (the other side of the coin; maybe THEY will replace the immortals). Chemical explosions just don't work. They burn, sure, and flare up... but flour-mill detonations don't happen, forest fires are a lot less tragic, and a peasant with a pistol can't assassinate Archduke Ferdinand.
Lethality in Empyrea is a matter of personal skill, tactics, and thought -- not just a lucky shot.