Alsahra' Alkubraa

As the throngs of Faithful make their neverending trek across Alsahara' Alkubraa (Arabic for "Great Desert"), they keep in mind that the desert is more than just a geographical formation; it is their Goddess, their caretaker, and a living being.  And, as living beings tend to do, the Great Desert grows.
The Great Desert is located on the southwest corner of Rodinia.  On this world, weather and winds typically travel from west to east.  As such, the winds pick up sand from the tops of the millions of sand dunes, carries it east, and drops it onto grassland, smothering it, and converting it into new desert.  This process is a slow but inexorable one.
The Great Desert's size is currently a mind-boggling 145 million square miles (almost 41 times larger than the Sahara Desert).  It is growing at a rate of approximately 60,000 square miles each year.  Calculated projections indicate that, within three milennia, all of Rodinia will be incorporated into one massive desert.  This means that, slowly, all other races and species will need to either adapt to a desert existence, or else perish and go extinct.
A prophet among the Faithful has predicted that, once the planet becomes one vast desert, several species of desert rainworms will evolve into gargantuan sandworms, and they will roam the Great Desert, eating any living creature they can find, locating it by the vibrations it makes.
Every night, when the Faithful stop and set up camp, their revered diviners, prognisticators, and seers consult the stars in the night sky and the wind blowing across the dunes, and determine the megacaravan's route for the next day.  They never make camp in the same place twice.  And, since the Great Desert is constantly growing, it is quite possible that the Faithful will never run out of new places to visit.

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