Short answers, pulled from the story.
Osiris was the ancient Egyptian god of the afterlife, the dead, resurrection, fertility, agriculture, and vegetation. He was depicted in mummy wrappings, wearing an atef crown, and carrying a crook and flail. He served as lord of the underworld and held the title Khenti-Amentiu, meaning Foremost of the Westerners.
According to Plutarch, Osiris's brother Set conspired with 72 accomplices to murder him, tricking him into a sealed box that was thrown into the Nile. Isis searched for and retrieved his body, but Set dismembered it and scattered the pieces. Isis reassembled the fragments, revived Osiris by magic, and conceived their son Horus; Osiris then became god of the underworld.
The annual festival at Abydos, recorded on the Ikhernofret Stela from around 1875 BC, lasted five days. It began with a mock battle led by the god Wepwawet, followed by a procession carrying Osiris's body by boat, a day of mourning, a night vigil of prayers, and finally a dawn ceremony declaring Osiris reborn and returning his statue to the temple.
Banebdjed was Osiris's soul, or ba, worshipped almost as a separate deity in the Delta city of Mendes. The name means roughly the ba of the lord of the djed pillar, which symbolised Osiris's backbone. A living sacred ram was kept at Mendes as the incarnation of this aspect, and upon death the rams were mummified and buried in a dedicated necropolis.
At death, a person faced a tribunal of 42 divine judges. Those who had lived according to Maat, the goddess of truth and right living, were welcomed into the kingdom of Osiris. Those found guilty were thrown to the devourer Ammit and faced complete annihilation; the Egyptian system included no concept of eternal torture.
The cult of Osiris and Isis survived at Philae until at least the 450s CE, long after imperial decrees of the late fourth century had ordered the closing of temples to pagan gods. Philae was the last major ancient Egyptian temple to be shut.