The initiatory drink of the Eleusinian Mysteries: barley, water, a breath of mint — and possibly something more. The Homeric Hymn to Demeter (~7th century BCE) places it at the ritual’s heart, and classicists have argued since Hofmann and Ruck’s 1978 proposal that ergot-infected grain may have turned the cup psychoactive. The argument remains a hypothesis; the substance is unconfirmed, and the chapter is careful to date it honestly.
The Mysteries chapter makes the point that it does not matter — and means it. The kykeon was not drunk on a lark. It came at the end of a long initiation: days of fasting and purification, an oath of secrecy, the hierophant presiding, the whole apparatus of Eleusis closed around the initiate like a hand. The drink was the last step of a carefully bounded channel, not the channel itself.
What the Mysteries guarded was not the recipe but the keeper — the structure of initiation that made the widest possible opening survivable. The unbounded version (same channel, no keeper, no oath, no grade) is the vine-swallower of Chapter 3 wandering into something he was not prepared to close. Architecture, not substance: what is attested is the three-point bound around the cup.
Sources. The Homeric Hymn to Demeter (~7th c. BCE) and the Eleusinian Mysteries; the ergot hypothesis of Wasson, Hofmann & Ruck, The Road to Eleusis (1978) — a hypothesis, the psychoactive content unconfirmed. Search: kykeon Eleusinian Mysteries Homeric Hymn to Demeter; Road to Eleusis ergot Wasson Hofmann Ruck.
Appears in: The Mysteries · The Vow