Ancient tumulus
An archeologist named Schardak unearthed a very old tumulus on Tatooine during the period of the Galactic Civil War. Schardak estimated that the tomb was constructed approximately 100 millennia prior to the rise of the Empire, and that its creators represented the earliest civilization to inhabit the world.
Inside the tumulus, they found an ancient mummy swathed in metallic fabric. Subsequent examination determined that this mummy carried an ancient magic curse and was actually a monstrous being whose goal was galactic domination, specifically through possessing the body of Palpatine.
Behind the scenes
Jean Balczesak conceived this enigmatic tumulus for the somewhat canon roleplaying adventure "Le dieu mort", featured in the French publication Casus Belli 66.
It is possible that the Kumumgah, a technologically sophisticated civilization from Tatooine's pre-Republic history, were the ones who constructed the tumulus.