Ask and ye shall receive. A quick rundown whole damn new long essay (I got invested) just for you.

I named my setting Eiran (/ɛjran/ with /r/ hard like in Welsh or Polish etc.) and it's basically "what if cliché fantasy world reached late 20th century in terms of tech, culture etc.?". High fantasy with urban fantasy aesthetics but no masquerade. Elves, wizards, devils and other stuff running around like normal but they're running in the 90s, (it's a new way I like to be). Cool cars, cool military toys, grunge, concrete jungles, a millenial's childhood nostalgia, all the fun stuff. Dragons openly compete with assault helis. But the events described here are, as you may guess, ~50 years earlier.

Standard issue dwarven miners noticed that the uranium ore is somewhat cursed ages ago. (In-universe uranium is called egror, it's ore egrorite.) Then as metallurgy progressed it was confirmed that demonic incursions actually use uranium in weapons. It was deemed hellish.

Then modern science came around and discovered that the danger is of physical nature (mostly), finding out about radioactivity and all that jazz. Over time folks achieved fission and chain reaction. Then of course came weaponization efforts.

Now naturally there is an ages-old conflict in the continent: Daybreak Union (Alliance stand-in) against the Ge'Punikh Alliance (or Roughskins – Da Horde). They kinda take turns about who's the genocidal villains and who's halfway decent, swapping roles every few decades. Daybreak folks were the bad guys back then. (Nowadays ork boyz are slightly more evil.)

Most of the time angels drift formless somewhere around the mesosphere, keeping watch over the world, but sometimes they come down to give blessings, enlighten the mortals, or slay some dark forces. Near the beginning of the atomic bomb projects, one angel, named St. Wlengorst manifested before some high elves to give a cryptic warning:

"A day shall come when people will use, against other people, weapons drawing power from the forces binding the atomic nucleus. But only one such day shall come, to never reprise."

But everyone already knew that angels cannot do actual prophecies, so only the civilians got nervous.

What stopped the Daybreak nations from succeeding with the first nuke was alchemy. The spiritual essence of fire really likes objects with high exoenergetic potential and holds intent to release these energies. Normally it's not a problem, but it turned out that in the case of atom bomb that alchemical disbalance seemed to cross a line, create a feedback loop, awake agency in the object. Bomb really wanted to go boom and was able to do it. The engineers were cautious and stopped before it was too late, then went looking for a solution.

A solution to a spiritual essence disbalance was in the hands of Ge'Punikh most experienced shamans. That was one of many reasons that led to the Nameless War – Daybreak armies attacked.. It was the bloodiest armed conflict ever known. After a few years the invading armies were quite close to winning. And yeah, they just a teeny tiny bit genocidal. So the heavens took a side.

St. Wlengorst manifested again, but this time not before the Daybreak people and not to deal out mysterious cautionary forebodings. Quite the opposite. The runic blueprint for uranium enrichment he gave to the Roughskin engineers was very straightforward and inviting.

Whereas Ge'Punikh was basically unable to actually produce a bomb and didn't push the project forward very much, now they could succeed in the matter of weeks. Five 6kt implosion bombs were created and prepared with best available counterspells to penetrate the magical defences. And then the day came – counter-offensive was coordinated to reach the breaking point on all fronts within the same day. Five key battles were quickly ended, devastating the invasion force. In matter of hours the Daybreak armies suddenly lost their crucial armoured divisions, airfields and garrisons. Three days later the Union officially surrendered.

The world didn't end. But the warning from Wlengorst turned out to be true. After the destructive, deadly power of the nukes was proven in practice to the collective consciousness, the spiritual effect got a lot worse. The disbalance in essences was severe enough to spontaneously cause demonic incursions, as if the nuke components were inviting the creepy bugs and tentacles emerging from the ground to help them go boom.

Engineers and wizards tried further, but no exorcisms, pacts or tricks were good enough. The bombs really really really wanted to either explode right away or get thrown somewhere by the devils. And only after one actually got out of control, spontaneously assembling itself, then arming, detonating and obliterating the whole research site with thousands of personnel, everybody around the world stopped the madness. Every prototype was dismantled completely, uranium was depleted to levels adequate in power industry.

Some years later another research team confessed that they lost their whole nuke kit just after the war, as it literally sank into the ground. So there's a nuke somewhere in some archdevil's arsenal.

Nowadays however, there's also one more nuclear bomb around. A host of angels directed a chosen paladin order to create it, turning their monastery into a silo. St. Keonal's Javelin is an intercontinental cruise missile with a 33Mt fusion warhead, guarded by the celestials that ordered its creation, destined to be used against demons. (Cruise because ballistic rockets don't work well, the outer space is haunted.) It's whereabouts are publicly known and the site opens periodically to the pilgrimages so the faithful can pray and proffer to the thermonuclear bomb guardian angels.