Short answers, pulled from the story.
The name Osiris is derived from the Egyptian word wsjr, which remains a subject of scholarly debate. Some Egyptologists suggest it means The Mighty One, while others argue it signifies the engendering male principle or the product of the ritual mummification process.
Osiris became the first mummy when his wife Isis used her magic to briefly revive him to conceive their son Horus before wrapping his body in linen bandages. This act of resurrection established Osiris as the god of the afterlife and marked a radical shift in ancient Egyptian theology.
The worship of Osiris began during the Fifth Dynasty of ancient Egypt. The cult continued at Philae until at least the 450s CE, long after imperial decrees ordered the closing of temples to pagan gods.
Set dismembered the body of Osiris into fourteen pieces and scattered them across the land to implicate his conspirators. Isis managed to retrieve all the parts except for the genitalia, which had been cast into the river and eaten by fish.
The festival of Osiris began on the same day grain was planted in the ground and involved the construction of Osiris Beds filled with soil and seed. Public plays depicted the dismemberment of Osiris and the search for his body, while priests performed esoteric ceremonies inside temples.
Osiris presided over a tribunal of forty-two divine judges where the fate of the deceased was decided by the weighing of their heart against the feather of Ma'at. If a person had lived in conformance with Ma'at's precepts, they were welcomed into the kingdom of Osiris, but if found guilty, they were thrown to the soul-eating demon Ammit.