(This world was effectively an experiment on how a fantasy world would react to a guy showing up outta nowhere with bad isekai main character levels of power, breaking the laws of reality permanently, changing how the universe works, and then leaving without elaborating, Fate-Killer barely shows up and it's genuinely funny.
There is thousands of years of lore before this, and a couple hundred after, this is comparatively a small piece of this world's story, and it's still somewhat ongoing, no I am not writing a book, or a show, this is for leisure only, and to get the whole picture you have to ask me, PLEASE ASK ME I'M FRIENDLY :) I'll awnser every comment probably.)
51 B.D. (Before Dawn) every prophet in the world foresaw the end of the world by the hands of the Demon King Katarnus.
Katarnus and demonkind in general had achieved effectively complete dominion of the northern lands, and were defeating almost every army they faced, everything seemed hopeless.
Until a nameless massively tall armored man, with a red cape and his dire-wolf, started a bloody 10 year trek north, to kill Katarnus.
Single-handedly, the Nameless Knight slaughtered demons at a scale never before seen, slaughtering entire hordes alone, zig-zagging across the continents to leave none behind.
The tales of this unknown knight single-handedly defying every odd imaginable spread like wildfire, inspiring many adventurers, including Gunter "Green-Hood" Lathander and his brother, to join the fight.
The Nameless Knight faced one of the 13 Sages, the strongest demons (excluding Katarnus) it was the only half-demon in the world, a woman called Lorelei, powerful enough to decimate armies single-handedly, and the Nameless Knight beat her with ease.
Discovering that, unlike all other demons, due to being half-human, Lorelei had emotions, and was horrifically abused, The Nameless Knight decided to take her into his protection, and they grew incredibly close.
There was a small hamlet at the edge of demon territory, Ardloth.
Gunter and a militia of adventurers from all over were brutally holding the line and keeping the town from being overrun by demon forces, but they knew they wouldn't hold for long.
Gunter's brother was slain in battle by the weakest of the 13 Sages, and the dead were piled in front of Ardloth as to make a wall to minimally increase protection.
Then, the Nameless Knight, riding his Dire-Wolf, rode through the middle of Ardloth, not saying any words, and not stopping for any reason, and headed straight for Katarnus's Fortress.
The sight of this legendary man sparked something in the warriors protecting Ardloth, so they gathered arms, and charged right behind The Nameless Knight.
The Nameless Knight entered battle far before any of the Ardloth Coalition did, and it was a slaughter.
Most of demonkind was established around Katarnus's fortress, they numbered in the hundreds of millions. And along with them were 11 of the remaining Sages, the strongest of them. 3 fled immediatly, the 3 weakest of those 11 fearfully(and correctly) assesed that they were going to die if they fought The Nameless Knight, and fled along with many millions of demons, becoming known as the Triumvirate forever after.
The Nameless Knight killed 85% of all demonkind that day, including the 8 Sages that dared stand in his way, and headed for Katarnus.
Then, the battle of Ardloth began, with the weakest of the Sages, along with a small legion of demons, met the Ardloth Defenders in battle.
The battle was fierce, but Gunter managed to sneak attack the Sage by pretending to be dead, and then cutting off the demon's staff-hand when the Sage floated past him, causing the dwarf bowman Kwurf to shoot the Sage dead through the eye.
The first clash of The Nameless Knight's sword against the Demon King’s blade ignited a fulmination so brilliant that the night sky, from one horizon to the other, turned to noonday blue for a full seven seconds. This was the opening of what is now called the Thousand-Hour Duel.
Every clash of swords caused this size of a flash, the powers clashing so titanic the entire world shook for the entire duration;
They fell into the deepest pits of the abyss, and throughout the duel, they battled their way up, out into the deepest depths of the ocean, then onto land, and finally, on the 41st day of the duel, the supposed last day of the world's life, they battled at the peak of Mount Nemmen, the tallest mountain in the world, right next to Katarnus's fortress.
It was night time again, and according to all prophets, no dawn would rise again.
But then, The Nameless Knight won.
A piercing strike went right through Katarnus's sword, into his eye, and out the back of his skull, killing him, for real.
Moments later, the dawn all prophets claimed would never come, arrived.
The Unwritten Dawn.
Immediatly after Katarnus's death, every prophet in the world lost their powers.
Fate itself, had been killed.
The Nameless Knight was celebrated and sung about across all the lands, but with a new name now...
Fate-Killer.
He then took over Katarnus's fortress and exclusively the supposed inhospitable environment, and decided to live there with his wife, Lorelei. And never made a public appearance again, other than to hire workers to fix up the Fortress.
His descendants were kept anonymous for their protection.
Now having the upperhand, the armies of the free lands hunted and slaughtered as many of the remaining demons as they could find.
And the Triumvirate and their millions of demons hid, for they knew, for as long as Fate-Killer lived, they were as good as harmless, and they had to wait him out.