The Great Dying occurred in 2081AD. At the time, nobody knew what caused it. Over nine billion people died. Statistically, that was all of humanity. Only a few million humans survived. We know now that many of them survived because they were innoculated against the artificial virus. Some survived because of natural immunity. Still others survived because they were so isolated, they never came in contact with the virus before it dissipated. So much death plunged mankind into utter chaos. The very fabric of society was ripped apart. Civilization devolved into nomads and farmers and remained that way for centuries. In fact, the Vuduri culuture marked its calendar as PR, Post Resurrection, as beginning from that date.
After several centuries, society needed an overseer, a guiding force, benevolent but with authority. The Cavaliers were created to be police, judge, jury and occasionally executioner to keep order among the people. The mantle of the Cavalier was handed down from father to son. Twice a year, they would go on the "Long Ride" to circle around their territory, making sure everyone was OK and keeping the peace.
It was only in the novel entitled The Ark Lords that we find out that the virus was designed, developed and released on purpose to "reboot" the Earth. More on that later.