The Egyptians were not satisfied with the mere possession of the texts of Thoth, when their souls were being weighed in the Great Scales in the Judgment Hall of Osiris, but they also wished Thoth to act as their Advocate on this dread occasion and to prove their innocence as he had proved that of Osiris before the great gods in prehistoric times. According to a very ancient Egyptian tradition, the god Osiris, who was originally the god of the principle of the fertility of the Nile, became incarnate on earth as the son of Geb, the Earth-god, and Nut, the Sky-goddess. He had two sisters, Isis and Nephthys, and one brother, Set; he married Isis and Set married Nephthys. Geb set Osiris on the throne of Egypt, and his rule was beneficent and the nation was happy and prosperous. Set marked this and became very jealous of his brother, and wished to slay him so that he might seize his throne and take possession of Isis, whose reputation as a devoted and loving wife and able manager filled the country. By some means or other Set did contrive to kill Osiris: according to one story he killed him by the side of a canal at Netat, near Abydos, and according to another he caused him to be drowned. Isis, accompanied by her sister Nephthys, went to Netat and rescued the body of her lord, and the two sisters, with the help of Anpu, a son of Ra the Sun-god, embalmed it. They then laid the body in a tomb, and a sycamore tree grew round it and flourished over the grave. A tradition which is found in the Pyramid Texts states that before Osiris was laid in his tomb, his wife Isis, by means of her magical powers, succeeded in restoring him to life temporarily, and made him beget of her an heir, who was called Horus. After the burial of Osiris, Isis retreated to the marshes in the Delta, and there she brought forth Horus. In order to avoid the persecution of Set, who on one occasion succeeded in killing Horus by the sting of a scorpion, she fled from place to place in the Delta, and lived a very unhappy life for some years. But Thoth helped her in all her difficulties and provided her with the words of power which restored Horus to life, and enabled her to pass unharmed among the crocodiles and other evil beasts that infested the waters of the Delta at that time.
When Horus arrived at years of maturity, he set out to find Set and to wage war against his father's murderer. At length they met and a fierce fight ensued, and though Set was defeated before he was finally hurled to the ground, he succeeded in tearing out the right eye of Horus and keeping it. Even after this fight Set was able to persecute Isis, and Horus was powerless to prevent it until Thoth made Set give him the right eye of Horus which he had carried off. Thoth then brought the eye to Horus, and replaced it in his face, and restored sight to it by spitting upon it. Horus then sought out the body of Osiris in order to raise it up to life, and when he found it he untied the bandages so that Osiris might move his limbs, and rise up. Under the direction of Thoth Horus recited a series of formulas as he presented offerings to Osiris, and he and his sons and Anubis performed the ceremonies which opened the mouth, and nostrils, and the eyes and the ears of Osiris. He embraced Osiris and so transferred to him his ka, i.e., his own living personality and virility, and gave him his eye which Thoth had rescued from Set and had replaced in his face. As soon as Osiris had eaten the eye of Horus he became endowed with a soul and vital power, and recovered thereby the complete use of all his mental faculties, which death had suspended. Straightway he rose up from his bier and became the Lord of the Dead and King of the Under World. Osiris became the type and symbol of resurrection among the Egyptians of all periods, because he was a god who had been originally a mortal and had risen from the dead.
But before Osiris became King of the Under World he suffered further persecution from Set. Piecing together a number of disconnected hints and brief statements in the texts, it seems pretty clear either that Osiris appealed to the "Great Gods" to take notice that Set had murdered him, or that Set brought a series of charges against Osiris. At all events the "Great Gods" determined to investigate the matter. The Greater and the Lesser Companies of the Gods assembled in the celestial Anu, or Heliopolis, and ordered Osiris to stand up and defend himself against the charges brought against him by Set. Isis and Nephthys brought him before the gods, and Horus, "the avenger of his father," came to watch the case on behalf of his father, Osiris. Thoth appeared in the Hall of Judgment in his official capacity as "scribe," i.e., secretary to the gods, and the hearing of the evidence began. Set seems to have pleaded his own cause, and to have repeated the charges which he had made against Osiris. The defence of Osiris was undertaken by Thoth, who proved to the gods that the charges brought against Osiris by Set were unfounded, that the statements of Set were lies, and that therefore Set was a liar. The gods accepted Thoth's proof of the innocence of Osiris and the guilt of Set, and ordered that Osiris was to be considered a Great God and to have rule over the Kingdom of the Under World, and that Set was to be punished. Thoth convinced them that Osiris was "MAA KHERU," "true of word," i.e., that he had spoken the truth when he gave his evidence, and in texts of all periods Thoth is frequently described as S-MAA KHERU ASAR, i.e., he who proved Osiris to be "true of word." As for Set the Liar, he was seized by the ministers of the Great Gods, who threw him down on his hands and face and made Osiris mount upon his back as a mark of his victory and superiority. After this Set was bound with cords like a beast for sacrifice, and in the presence of Thoth was hacked in pieces.
