Demeter: Mother of Kore
By far the most mysterious and least known of the Roman Mystery Religions, the Eleusinian Mysteries are also thought to be the oldest. Originally from Greece, they probably were absorbed into Roman Empire when Greece was conquered in the second century BCE. Like the other cults, as Christianity became the official religion of the Empire, they were phased out by the seventh century CE.
Despite the secretive nature of the cult, it included many prestigious members in its ranks, including several Roman Emperors -- particularly Augustus, Hadrian, and Marcus Aurelius.
The central myth of the Eleusinians involved the kidnapping of Kore (Persephone to the Romans) the Daughter of Demeter (Greek goddess of fertility) by Hades (God of Death) to be his wife. Demeter remained in mourning for her daughter, and plunged the world into winter until the other Gods forced Hades to create a compromise. Thus, half the year Kore is allowed to spend with her mother, and the other half with Hades -- explaining the existence of seasonal changes. Very little little else is known about the cult. However, scholars are aware that it possessed a definite hierarchy determined by ancestry in the family line of Eumolpides and Kerukes. Scholars also know that sometime before the Roman invasion it incorporated rituals of the cult of Agrai, a Near Eastern Goddess similar to Demeter. Central rituals involved the handling of symbolic materials and animals (particularly snakes) and dramatizations of the Eleusinian mythos. Other themes include cyclical rebirth, as well as animal sacrifice, food and drink, fasting and ritual asceticism.
The Concise Columbia Electronic Encylopedia, 3rd Edition
Marica Eliade, ed., Encylopedia of Religion, Kurt Rudolph, "Mystery Religions," 233