The Frost Fall was a long cold that held the southern reaches of the world for roughly a century and a half, from 8,840 to 8,695 BSD. Its cause was not weather. Pelus, the water layer of the Elemental Planes, drifted abnormally close to the material world and stayed there, pressed against the leyline membrane that holds the elemental stack apart from the living world. A seam rupture tears one channel open in a moment; this was the whole plane leaning in, slowly, and holding the lean for generations. Cold bled through wherever the membrane ran thin, and the south did not warm again until Pelus drew back.
The people who lived through it never knew what it was. The chroniclers of the early Great Expansion recorded a winter that would not break and the famines and migrations that came with it, and they reckoned it as foul weather on a scale no one had seen before. The drift hung no fire in the sky and loosed no flood to point at. It only made the world colder, season over season, until cold was simply the condition of the age. The reading that names Pelus is a modern one, a Kethic case assembled centuries later and worked backward from the cold the drift left behind in the leylines.
The cold still in the ground
When Pelus withdrew, the cold went with it, but the leylines it had pressed against did not settle back to true. Cold-Kethic drawn over that southern ground runs treacherous. Pull water toward ice across those leylines and the working bites deeper and colder than the feeling behind it, locking faster than you meant or setting hard and refusing to release. That fault outlasted the drift by thousands of years, and it is the part of the Frost Fall still unfolding now. It is also why the cold places of the south hold their cold-Kethic so densely: the drift left the element pooled close beneath them, and it never fully drained.
A grain-pit in the southern lowlands, dug during the cold and never reopened, a stone-lined shaft packed with seed against a spring that came late for a hundred winters running. The grain at the bottom is still whole. The cold the drift left in that ground has kept it from rotting since Pelus withdrew, and the leyline beneath the pit still reads colder than the rock around it has any reason to be.
Shoryaven's Chronicle of the Great Expansion gives the Frost Fall its fullest surviving account, following the cold through the failed harvests, the famines, and the southern peoples who moved or died where they stood. He treats it as the first hard test the recovery passed. He names no cause for it, because in his day there was none to name.