The Oblivion Years were the eleven centuries between 11,500 and 10,400 BSD in which the world stopped being able to remember itself. The cause was not war and not plague. A leyline convergence in which Nilus ran strongest, the void layer of the Elemental Planes, lost its balance and came apart. Convergences hold several planes pressed close in one place, and a void-heavy one carries the canceling principle at its center; when this one destabilized and collapsed, what bled loose was not a substance but the lack of one. Void takes presence out of a space rather than putting anything into it. What it took out here, slowly and across a whole population, was coherence: the structure in the spirit that lets a person carry a continuous self and a continuous memory from one year into the next.
This is not the poisoning that killed the mages a few centuries earlier. The Blight ran lethal through the Ezz a working drew on; this thinned the part of the spirit that holds a self together, and it did not kill. People simply came loose from what they had been. A name stopped meaning the person who carried it. A craft learned over years drained out of the hands that knew it. Within a generation, a community could no longer reach back past the living, not because its records had burned but because the faculty that keeps a record meaningful had worn through.
No one inside the Oblivion Years grasped what was happening to them, and by the nature of the thing none of them could have. The survivors of the preceding Dark Night kept no written record and no oral tradition that ran back more than a generation, and the world began again from nothing. That the cause was a void convergence is something the present worked out, not something the era recorded: a Kethic reconstruction traced back from the one mark the Oblivion Years could not erase, the instability still lying in the ground where the convergence stood. No account from inside the Oblivion Years names the cause, because the Oblivion Years are the span in which the capacity to keep such an account came apart.
The coherence still in the ground
When the convergence collapsed, the void went out of the air, but the leylines it had run through never settled back to true. Void-Kethic refuses to settle over the old convergence. A shaper laying an absence there finds it cutting wider than aimed, canceling more than was asked and never quite closing. The damage has outrun the collapse by thousands of years, and it is where the Oblivion Years are not yet finished. It runs the other way too. People who cross the old convergence lose their grip on continuity while they stand on it, the order of their own days coming loose, names and intentions thinning the longer they stay. Where the convergence actually lay is no longer known, because the record that would have placed it is exactly the sort of thing the Oblivion Years took.
A line of survey stakes driven into a low plain somewhere in the old interior, set by a modern expedition that meant to map the dead convergence and never published its findings. The stakes stand in no order anyone can now reconstruct. The party's three field journals each give a different count of the days spent on the ground, a different route in, and a different reason for turning back, and not one of the three names the other two. The leyline beneath the plain reads thin, the way a thing reads when most of it has been taken out and never put back.