Iakkhos, the Ritual Cry of Eleusinian Mysticism and War

At this stage of the struggle, just before the historical sea battle of the Persians and Greeks, they say that a great light flamed out from Eleusis, and an echoing cry filled the Thriasian plain down to the sea, as of multitudes of men together conducting the mystic Iakkhos in procession. Then out of the shouting throng a cloud seemed to lift itself slowly from the earth, pass out seawards, and settle down upon the triremes.

Plutarch, Life of Themistocles