Short answers, pulled from the story.
The first evidence of the worship of Osiris appears in the middle of the Fifth Dynasty of Egypt during the 25th century BC. This timeline suggests that people may have worshipped him much earlier than this recorded date.
John Gwyn Griffiths proposes the name means The Mighty One based on the root wsr. Other theories suggest it refers to a seat of the eye or something made through ritual mummification while James P. Allen reads the word as jsjrj deriving it from an engendering male principle.
Set conspired with 72 accomplices to kill his brother Osiris using a trick involving a coffin. He lured Osiris into the box before sealing it shut with lead and throwing it into the Nile River.
Isis searched for the remains until she found them embedded inside a tamarisk tree trunk supporting a palace roof in Byblos. She removed the coffin and retrieved her husband's body from the wood.
Annual ceremonies began at Abydos on the same day grain was planted in the ground. The festival lasted five days and included public drama depicting the murder and dismemberment of the god.
The cult of Isis and Osiris continued at Philae until at least the 450s CE. This persistence lasted long after imperial decrees ordered the closing of temples to pagan gods in the late fourth century.