Last March of the Fair Folk
In the first weeks after the Fair Folk reappeared, there was panic in the streets. From London to New York, from Vancouver to Syndey, from Beijing to Jerusalem, the fey had returned. The reasons for their hostility towards humanity are still unclear. Some said it was for forgetting them. Others claim it was for ecological disruption. Some said they really just hated us.
Whatever the reasons, the beasts of legend and folklore now hunted humans. Ogres and giants smashed through tanks and armored cars as though they were toys. Pixies swarmed hapless victims and left them to die from a thousand cuts. Hobgoblins at night in homes, killing entire families in their sleep and spreading terror. Changelings would turn against their "parents" without warning.
The ability of all of them to shrug off gunfire, explosives, flamethrowers, and almost all conventional weapons was of secondary concern to the panic they caused. There were stories of a handful of brave humans fighting off the fey with iron weapons, but they were almost always overwhelmed. Humanity was losing a war of survival against supernatural invaders.
It was the second month after the initial invasion that all of that changed. A research team in Austria devised a prototype coil-gun. It was a semi-automatic rifle that fired unworked iron projectiles using an electromagnetic coils around the barrel. A military version was rushed into mass production around the world, alongside grenades loaded with iron shrapnel. The vulnerability of the fey to iron did the rest. By the end of the year, the Seelie and Unseelie Courts had unconditionally surrendered. They got off easy. Historians would call it the Curb-Stomp War.