Mirza Mihdi, designated Ghusnu'llahu'l-Athar (The Purest Branch) by his Father, was the second surviving son of Baha'u'llah. He was the full brother of 'Abdu'l-Baha (Ghusnu'llahu'l-A'zam: The Most Great Branch) having the same mother, Navvabih Khanum. In 1870, he was twenty-two years old. It was his wont to go in the evening to the roof-top of the citadel to pray and meditate. There one gets a wonderful view of the pellucid blue of the Mediterranean, with the silhouette of Mount Carmel beyond the seascape; and to the other side lies stretched the plain of 'Akka with the majestic peak of Mount Hermon in the background. One evening, Mirza Mihdi, pacing up and down that roof-top engrossed with his thoughts and meditations, did not notice an open skylight and plunged through it to the floor below, falling upon a crate which pierced his chest. The injury proved fatal.
Aqa Husayn-i-Ashchi recalled that the sound of his fall and the rush of the companions towards him brought Baha'u'llah from His room. He anxiously enquired what had happened. The Purest Branch said that he had always counted his steps to that skylight but on that evening had forgotten to do so. An Italian physician was called in, but his treatment was of no avail. Although obviously suffering, the Purest Branch remained attentive to his visitors, the companions who came to stand or sit at his bedside and to attend to his needs. Aqa Husayn remembered that he would express his unease at having to lie down in their presence. Within twenty-two hours of his fall he breathed his last. Aqa Husayn recalled hearing Baha'u'llah lamenting aloud: 'Mihdi! O Mihdi!' He also recalled that before death overtook the Purest Branch, Baha'u'llah asked him: 'Aqa, what do you wish, tell Me', to which His son replied: 'I wish the people of Baha to be able to attain Your presence.' 'And so it shall be,' Baha'u'llah said; 'God will grant your wish.' The day of his death was 23 June 1870 (23 Rabi'u'l-Avval AH 1287).
The Guardian of the Baha'i Faith writes:
His dying supplication to a grieving Father was that his life might be accepted as a ransom for those who were prevented from attaining the presence of their Beloved.
In a highly significant prayer, revealed by Baha'u'llah in memory of His son - a prayer that exalts his death to the rank of those great acts of atonement associated with Abraham's intended sacrifice of His son, with the crucifixion of Jesus Christ and the martyrdom of the Imam Husayn we read the following: "I have, O my Lord, offered up that which Thou hast given Me, that Thy servants may be quickened, and all that dwell on earth be united.' And likewise, these prophetic words, addressed to His martyred son: 'Thou art the Trust of God and His Treasure in this Land. Erelong will God reveal through thee that which He hath desired.”(Shoghi Effendi, God Passes By, p. 188) (Balyuzi, ‘Baha’u’llah, The King of Glory, pp. 312-313)