The Rift War was the clashing of ideas between members of the Alstromerian royal family who were pro and anti-necromancy, with the latter seceding and becoming the kingdom of Marchosia. An unexpected consequence was a loose organization of non-magic users rebelling from Alstromerian rule during a moment of weakness and forming the kingdom of Eucyon.
This violent conflict resulted in the destruction of many cities and the loss of many lives. The destruction of Atlaxia is often used in art and song as a symbol of the war. To help bolster their numbers, Alstromeria built advanced golems known as warforged, while the proto- Marchosians summoned the shadar-kai, a warrior race from the Shadowfel.
Fortunately, the Rift War ended up benefiting everyone (or at least everyone still alive). All races can now freely practice magic. All of the current kingdoms consider themselves to have come out winners. Alstromeria is absent the necromancers, Marchosians can practice their death magic in peace, Eucyon is free of magocratic rule, the Uschalian City States have regained their self-governance, and Kaliphora is making a mint off of the other kingdoms’ trade embargoes against one another.