The Myth of Osiris and Isis, concerning the deities of Egyptian mythology Osiris, Isis, Horus, and Set, became one of the most important and powerful in Egyptian mythology during the New Kingdom. The myth concerns the death of Osiris and the birth of Horus.
The coffin and 14 parts
The original form of the myth states that Osiris was killed by a wooden sarcophagus secretly being made to his measurements by Set, who was jealous of Osiris's position as king, and so plotted to kill him and take his place. A party had been held where the coffin was offered to whoever could fit inside. A few people tried to fit in, but to no avail. Osiris was encouraged to try, but as soon as he lay back, the lid slammed on him and was locked. It was then sealed with lead and thrown into the Nile. Upon hearing that Osiris was gone, Isis set out to look for him. She was afraid without proper ceremonies and burial Osiris would not be able to go to the place of the dead. She later learned that the coffin had floated down the Nile river up to the coast of Byblos (now in modern day Lebanon) and got embedded in the trunk of a cedar tree. She also learned that the cedar tree had been taken and used as a pillar to support a palace for the king of Byblos. When traveling back, along the Nile River, she left the coffin in an area of marshland. Set, while hunting, finds Osiris' coffin and dismembered him into 14 parts, scattering them across the land of Egypt. Each part represented one of the 14 full moons (each year has 12 to 14 full moons).
Once again Isis set out to look for the pieces and she was able to find 13 of the 14 parts, with the help of Nephthys, Set's sister-wife, but was unable to find the 14th, as it had been eaten by a fish. Instead, she fashioned a phallus out of gold and sang a song around Osiris until he came back to life.
Osiris was resurrected. He could have proper ceremonies and burial.
Due to this experience, Osiris became Lord of the Dead, and the Afterlife. 
The moon and the oxyrhynchus
In late Egyptian thought, the righteous dead were sometimes said to become the stars, and thus the moon was occasionally seen as having a connection to Osiris, lord of the dead. As a death and resurrection legend, in which evil seeks to destroy a deity, thus bringing darkness, it thus developed an association with the lunar cycle, in which the moon appears to be destroyed by darkness, and is then brought back to life. Thus it later became said that Osiris had been killed by being dismembered into 13 parts, each part representing one of the 13 full moons seen each year (there are roughly 13 lunar months per year). Another interpretation is that the pieces were 14 (they number up to sixteen in some versions) were the phases of a single moon's cycle (one sliver cut off each night for 14 days, then reassembled over the next 14 days—see Janet McCrickard, Eclipse of the Sun, 1990). The original form of Set's murder of Osiris was incorporated into this later version, though it was said that the attempt had failed when Isis and Nepthys found the coffin and rescued it.
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