The old Shemsu-Heru called Ra-Hekau the supreme art of Hekau, the mastery of the world by using the True Names of everything to bend it to one's will. According to egyptian legend, Ra-Hekau was first revealed when the Sun-God Re gave Isis his True Name so that she could heal him from a deadly poisoning. It allowed Isis to become the Goddess of magic and allowed her to construct the Spell of Life.
The Shemsu-Heru believed that, once they had mastered the magical arts to a great enough degree, they would be able to accomplish similar feats. The Shemsu-Heru were mistaken. Ra-Hekau is the art of True Magick that can only be used with an Awakened Avatar. Since any Avatar they had once possessed passed on since they had lived the First Life, no mummy is able to bend reality to their will like mages can.