The Hom-Dai curse was an ancient malediction created by the ancient high priests of Egypt as a punishment reserved for blasphemers. It is described as "the worst of all ancient Egyptian curses. One so horrible, it had never before been bestowed."
The Hom-Dai was used by the ancient Egyptians only to punish the worst blasphemers that committed severe crimes, and so was used very rarely, if at all. Although the curse was mentioned in text, there was no written case of it ever actually having been performed, because the Egyptians greatly feared it, and the sole known case of the Hom-Dai ever being carried out was on that of the High Priest Imhotep, who committed the crime of using the Book of the Dead to bring back his lover, the Pharaoh Seti I's mistress, Anck-Su-Namun, from the dead. The Hom-Dai was meant so potent a curse that should any of its victims ever arise, with them would come forth the Ten Plagues of Egypt, as God himself had set unto Egypt in biblical times. The victim would also be condemned to a cursed life that would not end, even in death. And although a person cursed with the Hom-Dai might be physically dead, they were still capable of being brought back to life through an incantation from the Book of the Dead.
Bound to the victim's existence was a chest that, should it ever be opened, would leave those that opened it bound to the curse of the Hom-Dai, as the original victim would seek out whomever was present when the chest was opened, so as to kill them and assimilate their organs and fluids. By doing this, the original Hom-Dai's victim would regenerate themselves to their original state of being, and no longer be undead, but a being with unspeakable powers and immortality. Until the victim is fully regenerated, they will fear the presence of cats, who are the guardians of the underworld in Egyptian mythology.
The Hom-Dai was prepared by first cutting the person's tongue out. They were then mummified alive with scarabs, which would eat them very slowly and painfully. Reading an incantation from the Book of the Dead resurrects the cursed individual, giving them the power of invincibility, but condemning them to be a spreader of pestilence. A passage from the Book of Amun-Ra removes the immortality from the cursed individual, leaving them vulnerable and subject to being killed.
"The cursed victim, if brought back, would return a walking disease, a plague upon mankind, an unholy flesh eater, with the strength of ages, power over the sands, and the glory of invincibility. "
Based on several scenes in the first film, a victim of the curse could affect the mortal realm of Egypt in subtle ways, such as manipulating the sands and creating ominous noises to frighten away people and animals.