Opening: Tribe leaders waged destructive wars for houndreds of years, mainly because they couldn't kill eachother - instead they tried to imprison their powerful enemies. When destruction of planet they caused became critical, orginal creator of the world intervened and banished those powerful beings to other planes.
Take into account you're citing chronicles from primitive times, we're 120.000 years further down the line. These white ash issues have been under thorough research by historians and archeologists. ItThis peculiar ground layer has been a scientific mystery for many years. Recently, the remnants of a steampunker superstructure were found on Mespa, the neigbouring planet. Now the real story can be told.
Planet got disinfected. A neighbouring planet, inhabited by colonists who fled these wars, set up an operation to do that, because the two parties started to use a biological weapon that could spread through space. As a preventive measure, the planet was completely wiped clean using a gamma ray, taking out the larger cities first.. and then scanning over the planet surface, killing the virus, its makers and anything that could have gotten infected. After a single day, only white ash remained.