The World Fire was a catastrophic rupture of the Yolus leyline, the seam through which the Kethic element of fire reaches the material world. It happened around 10 BSD. The calendars that outlasted it set that year at zero, and it is still the boundary between the BSD reckoning and the SD age that came after.
A single fire-paragon caused it. Working a stretch of the Yolus seam where the element pressed close and ran strong, the paragon reached for a working larger than any feeling they could sustain. Fire-Kethic burns on the practitioner's own emotion, and the strong seam amplified that draw past what the self could supply. When the demand outran the feeling that fed it, the seam tore. A controlled channel became an open wound, and fire bled out of Yolus into the world at a scale no shaper intended and none could call back. It is the plainest lesson every fire-shaping tradition since has kept: the element will take more than the will holding it, if the will reaches too far.
The burned country
The fire ran for a long time and across a great deal of ground before it spent itself. Where the worst of the spillover settled, the land never came back.
The country at the center carries no name on any map drawn since. The ground there fused to a dark glass that rings faintly underfoot, sheeted over what had once been fields and a town or two, and little grows thick on it even now. Those who cross it say the air holds its warmth long after dark, with no fire anywhere in sight to account for it.
The seam that never closed
The Yolus seam did not heal over once the burning stopped. It runs erratic where it ruptured, surging and guttering on no schedule a shaper can read, so fire-Kethic worked over that ground behaves badly: a small flame swelling into something far larger than asked, or dying mid-working for no cause. This is the part of the World Fire that has not yet gone out. A shaper who works fire over that ground takes a real risk now, centuries after the burning stopped.
The lost center
No one knows where the paragon stood when the seam tore. The people nearest the working were the ones who understood what it was, and they died in its first moments; whatever had been written down about the seam or the method burned with the country around it. So the cause is certain and the place is not. What is gone is the location and the working that opened it, not the question of whether anyone was there to be caught.
They were. The Cinderbound, the ash-bodied beings that still keep to the burned country, are what became of those who stood closest to the rupture and were carried out of it as something other than dead. They are its living remnant, and the proof that the center was inhabited. Their own account belongs to their own entry.
The age that followed is reckoned from the year the burning ended, which is why the Seventh Dawn counts from zero.