Short answers, pulled from the story.
Osiris was murdered by his own brother Set, who then usurped his throne, according to the Osiris myth. Egyptian sources give varying motives, including revenge for a kick or a grievance over Osiris's affair with Nephthys, and never describe the killing directly.
Isis searched for and restored her husband's body, often with help from Thoth and Anubis, making Osiris the first mummy. She then conceived their son Horus from his restored body, though his revival was not permanent and he became ruler of the Duat, the realm of the dead.
Horus was born and raised in a thicket of papyrus in the Nile Delta called Akh-bity in Egyptian, meaning the papyrus thicket of the king of Lower Egypt. Greek writers called the place Khemmis and located it near the city of Buto.
The Osiris myth first appears in the Pyramid Texts, spells carved on pyramid burial chamber walls at the end of the Fifth Dynasty, in the 24th century BCE. Later versions appear in the Coffin Texts of the Middle Kingdom and the Book of the Dead of the New Kingdom.
Horus challenges Set for the throne of Egypt to avenge his father Osiris's murder and claim rightful kingship, a dispute argued before the assembled gods of the Ennead. The conflict includes racing contests, mutual mutilation, and, in one account, sexual assault by Set.
Plutarch wrote the most complete ancient account of the Osiris myth in On Isis and Osiris in the early 2nd century CE, addressed to a Greek priestess of Isis. His version adds episodes absent from Egyptian sources, including Set's trick chest and the tree at Byblos that grows around Osiris's coffin.