A great hall in Eleusis, Telesterion was one of the primary centers of the Eleusinian Mysteries. At some point in the 5th century BC, a man named Ictinos built the Telesterion big enough to hold thousands of people.
As 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). This was the most secretive part of the Mysteries and those who had been initiated were forbidden to ever speak of the events that took place in the Telesterion.
It was destroyed by the Persians and was subsequently rebuilt some time later by Pericles.