A dumb #dnd quibble; the evil elemental earth cult is the Cult of the Black Earth.
But “the land of the black earth” here is KMT. Not only is it kinda not cool to have one more evil thing be “black,” but to Egyptians evil was associated with “The Red Land.” 5e defaults to the Realms and the Mulhorandi are basically Kemetics, so why not make ‘em a Red Earth cult?