Rehu is the progressive story of a world. From the mythical Creation Wars to the terrible Death of the Rain, the major historical events of Rehu are lived through separate campaigns and the impacts of each can be seen as the world advances through time. Run with the same party for five years, each story arc represents a different era and the players are given the chance to influence the world in their own unique ways.
For example: In the Creation Wars, the mythological founding of the world, the party members don the mantles of the five Primarchs- progenitors of the races and demigods in their own right. Each is now revered by their respective progeny and the players get to see the way their particular characters influenced the culture, identity, and mythology of their race.
This world is the culmination of thought and writing for almost a decade. Starting off with a really weird dream and spiraling into something vast, messy, and wonderful. I haven't run a D&D campaign outside of it since college, and the timelessness and open-ended design allows for any number of unique stories to emerge without threatening the fabric of the world.