Short answers, pulled from the story.
The Great Ennead consists of Atum, Shu, Tefnut, Geb, Nut, Osiris, Isis, Set, and Nephthys. These figures formed a divine family tree where Atum sat upon a mound rising from the primordial waters of Nun. Horus the Elder sometimes joined these nine figures in ancient texts.
The Great Ennead first appeared during the Old Kingdom period before 2300 BCE. It thrived until the Ptolemaic era ended Egyptian independence on the 30th of May 30 BCE. Ra's cult dominated early periods under Pharaoh Userkaf of Dynasty V while later rulers like Pepi II saw Ra's influence wane.
Memphis priests declared Ptah superior because they presented a different cosmogony where Ptah spoke the world into existence. This regional competition prevented any one ennead from achieving universal acceptance across ancient Egypt. Each nome maintained its own local deities who demanded equal or greater status than the Heliopolitan group.
Atum emerged alone from Nun through self-creation rather than external forces according to Heliopolitan texts. He either spat or masturbated to produce Shu and Tefnut as air and moisture personified. The mound on which he sat marked the First Occasion of creation itself.
The English word Ennead derives from Latinenneas meaning nine while Greeks used enneás to describe groups of nine deities. Egyptians wrote the name with uncertain vowel sounds recorded in hieroglyphs and transcribed it conventionally as Pesedjet for scholarly discussion. No single pronunciation remains definitively known due to lack of vowel notation in ancient scripts.