The Mummies are an faction in Arenith led by the Inquisitor, deity of Sand. Years before the mummies never left the desert, held there by their ancient curse from the Lich King.
During the old period, the Mummies had a complex curse, stronger than other types of undead. They were simply led by the Inquisitor. They did get released from Battle of The Desert, causing the desert to be free.
Now, The Mummies have returned, with the help of a deity who was their leader from long ago. The inquisitor's new title had helped them by not only letting them resurrect, but let them partially become apart of the desert itself, Improving them.