Telesterion

τελείω, "to complete, to fulfill, to consecrate, to initiate") was a great hall and sanctuary in Eleusis, one of the primary centers of the Eleusinian Mysteries.

[5] At some point in the 5th century BCE, Iktinos, the great architect of the Parthenon, built the Telesterion big enough to hold thousands of people.

Then only a few hundred years later in 396 CE, the forces of Alaric the Visigoth invaded the Eastern Roman Empire and ravaged Attica, destroying the Telesterion, which was never to be rebuilt.

[3] At the climax of the ceremonies at Eleusis, the initiates entered the Telesterion where they were shown the sacred relics of Demeter and the priestesses revealed their visions of the holy night (probably a fire that represented the possibility of life after death).

Demeter gets angry, and tells her that since she robbed her son of immortality and angered her, the people of Eleusis must create a temple to her where they would do things to gain back her favor.

[6] Some temple use ceased during the persecution of pagans in the late Roman Empire, when all non-Christian sanctuaries were ordered closed by law initiated by the Christian emperors.