The Hom-Dai curse was an ancient malediction created by the ancient high priests of Egypt as a punishment reserved for the worst blasphemers.
The Hom-Dai was used by the ancient Egyptians only to punish the worst blasphemers that committed sexual crime with the pharaoh's daughter and sons, also the worst acts, 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 acted out, because the Egyptians feared it so, the sole known case of the Hom-Dai being carried out was that of the High Priest Imhotep, who committed the crime of using the black Book of the Dead to bring back his love, the Pharaoh Seti I's mistress, Anck-Su-Namun, from the dead.
The Hom-Dai was meant as so potent a curse that should any of its victims ever arise, with them would come forth all the Ten Plagues of Egypt, as God had set unto Egypt in Biblical Times. The victim would be condemned to a cursed life that would not end even in death, as although a person cursed with the Hom-Dai might be physically dead, they were still capable of being brought back to life.
Bound to the victim's existence was a chest that, should it be opened, would leave those that opened it bound to the curse of the Hom-Dai, as the original victim would seek 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, and no longer be an undead, but a being with unspeakable powers and immortality.
The Hom-Dai was prepared by first cutting the person's tongue out, the embalmers tilting the victim's head back so that he or she would not drown in their own blood. The person was then mummified alive with scarabs, which would eat him very slowly over time. Reading from the Book of the Dead would revive the cursed individual, giving them the power of invincibility, but condemning them to be a spreader of pestilence. A reading from the Book of Amun-Ra would take the immortality from the cursed individual, leaving them vulnerable.