Siyah-Chal - Baha'u'llah's Declaration

In the Síyáh-Chál, God made known to Bahá’u’lláh His great Station. Wrapped in gloom, breathing the foulest of air, His feet in stocks, and His neck weighed down by a mighty chain, Bahá’u’lláh received the first stirrings of God’s Revelation within His soul. Under these dreadful circumstances, the “Most Great Spirit” revealed itself to Him, bidding Him to arise and speak forth the Word of God.
At times, He would feel as if something flowed from the crown of His head over His breast, as a mighty torrent falls upon the earth from the summit of a high mountain. He saw the Maiden of Heaven suspended before Him, speaking to His inner and outer being, referring to Him as the Best-Beloved of the worlds, the Beauty of God, and the power of God’s sovereignty. He was assured that He would be made victorious by Himself and by His Pen, and by the aid of those whom God would raise up.
Thus from behind the darkness of the Black Pit rose the Sun of Truth. The Báb’s promise had been fulfilled. The Bahá’í Revelation was born. Yet Bahá’u’lláh did not inform anyone of what had occurred. He would await the appointed hour, ordained by God, to make His Mission known.
(Ruhi Book 4, p. 101-102)