The Great Plan of the Old Ones is the cosmic design left by the vanished god-beings who shaped the world, an inscrutable scheme that the Lizardmen exist to fulfil. Inscribed upon plaques of gold and stone, its edicts guide every action of the Slann Mage-Priests, who labour across millennia to see it done.
The Old Ones came in ages past to order the raw world, raising continents, guiding the stars, and creating the cold-blooded servants who would enact their will. Then the polar gates collapsed and Chaos flooded in, and the Old Ones departed or perished, leaving their plan half-finished and their creations to carry on alone. Ever since, the Slann pore over the sacred plaques, deciphering commands whose purposes span epochs.
To the Lizardmen no cause is holier, and they will scour a city or drown a continent if the Great Plan demands it. That much of the design was lost when the gates fell only deepens their fervour, for they believe that restoring the intended order of the world is the sole path to salvation from Chaos.