Alluded to from c. 2400 BCE; told as a connected narrative only c. 100 CE
The murder of Osiris
The one Egyptian myth everyone can name, and the one where the famous version is Greek.
Osiris rules Egypt as a king. His brother Set kills him, takes the throne, and disposes of the body. Isis, his sister and wife, searches for him, recovers him, and by her own magic revives him long enough to conceive a son. Osiris does not return to the living: he becomes king of the dead instead. Isis hides in the Delta marshes and raises the child, Horus, who grows up to claim his father's throne from his uncle.
That is the whole of it as the Egyptian sources have it, and they never tell it in one place. The Pyramid Texts of the Fifth and Sixth Dynasties assume the audience knows the story and refer to it obliquely, in ritual formulas addressed to a dead king who is being identified with Osiris. Isis and Nephthys mourn. The god is 'found'. The son avenges. Nothing is narrated, because none of these texts are narratives: they are spells to be spoken, and you do not explain a thing to the god you are speaking to.
The details everyone knows come from somewhere else. The chest that Set tricks Osiris into lying in, which is then sealed and thrown into the Nile; the pillar in Byblos that grows around it; the body cut into fourteen pieces and scattered; the piece a fish swallowed, so that Isis had to make a replacement: all of that is Plutarch, writing in Greek in the second century of the common era.
Where it comes from
The Pyramid Texts
About this sourcec. 2400 BCE. Refers to the murder constantly and never narrates it. Set is not always the killer in the oldest layers.
The Great Hymn to Osiris, stela of Amenmose
c. 1400 BCE. The fullest Egyptian account, and still allusive: it praises rather than recounts.
Plutarch, On Isis and Osiris
About this sourcec. 100 CE. The only complete connected version that survives, written in Greek by a Greek priest of Apollo, and the source of nearly every modern retelling.
What is late, foreign or single-source
The chest, Byblos, the dismemberment into fourteen parts and the swallowed phallus are Plutarch and are not attested in this form in Egyptian sources. Egyptian texts do refer to the body being scattered and gathered, so the idea is not his invention, but the number, the fish and the sequence are. Anything found only in Plutarch should be attributed to Plutarch.
What the story was for
Succession. The living king is Horus and the dead king is Osiris, so every royal funeral re-enacted this: a son burying a father and inheriting from him, with the transfer of power framed as a cosmic necessity rather than a political event. Its second job was to democratise the afterlife. In the Pyramid Texts only the king becomes Osiris. By the Middle Kingdom any Egyptian who could afford the rites could, which is one of the most consequential changes in the history of religion.
Other myths