Short answers, pulled from the story.
The Eleusinian Mysteries were annual secret initiation rites for the cult of Demeter and Persephone, held at the Panhellenic Sanctuary of Eleusis in ancient Greece. They were considered the most famous of the secret religious rites of ancient Greece and continued for roughly two millennia.
Men, women, and slaves were all permitted to participate, provided they were free from blood guilt (had never committed murder) and could speak Greek. Around 300 BC, the state of Athens took control of the Mysteries from the Eumolpidae and Kerykes families, leading to a vast increase in the number of initiates.
The Mysteries reenacted the myth of Persephone's abduction by Hades, Demeter's grief-driven search, and Persephone's eventual return to her mother. This cycle of descent, search, and ascent symbolized the eternity of life flowing from generation to generation.
The kykeon was a ritual drink made from barley and pennyroyal consumed by initiates before entering the Telesterion. Several scholars have proposed it contained psychedelic compounds from ergot fungus, citing ergot fragments found in a vase and in the dental calculus of a 25-year-old man at the Mas Castellar site in Girona, Spain.
The Eleusinian Mysteries were closed in 392 AD by the emperor Theodosius I, as recorded by the historian Eunapius. In 396, the Goth king Alaric I looted the remaining shrines during his raiding campaign in Attica.
Notable initiates included Aeschylus, Plato, Cicero, Sophocles, Augustus, Hadrian, Marcus Aurelius, and the emperor Julian, who reigned from 361 to 363 and was the last emperor to be initiated. Marcus Aurelius was also the only lay person ever permitted to enter the innermost sanctuary, the anaktoron.