Horizon is a novel from the Course Books Cannon that deals with events outside the Course Books story arc. The story is set on a distant Flat Dimension on the edge of the Known Worlds. It is also implied that this dimension may include the land of Rowen.
Within the book lots of dates and times are speculated on, however being outside the Known Worlds the characters do not have access to the Alliance Calender and are ultimately unsure of the events of the distant past.
In the timeline established by the Course Books, Horizon takes place during the Fifth Age. The 'conflagration' spoken of during the story is of course the Mage Wars; the Upheaval was a violent earthen wave.
The Oncar had discovered Iname ruins under their land, and were close to unlocking the secret of the Straw Men. This had given them access to powerful magic that made them quite a force to be reconed with in those days. It was well-known at the time that the Oncar were searching for ancient weapons of some sort, and that they were close. Using what they found in the Iname and Straw Man ruins, the Oncor were on the verge of discovering the massive fleet of buried SkyShips the Straw Men had built.
With this fleet, the Oncar could easily have conquered the Known Worlds, so rather than risk them finding the weapons, the much larger, much meaner Marcon Alliance simply sent a massive earthen-wave spell at the Oncar Empire.
The attack caused massive tectonic shifts that turned the relatively flat Oncar Empire into a treacherous world of high mountains and deep valleys, and also killed most of the Oncarins straight up. The few that survived went through a lengthy Age of Darkness and didn't get around to recording their history until the Second Age, by which time basically everything about the Oncar had been forgotten. .