The invocation “Yá Iláha’l-Mustagháth” (“O Lord of the time of ‘Mustagháth’”), or Yá Alláhu’l-Mustagháth” (“O Thou God Who art invoked”)

As Mr. … is no doubt well aware, the phrase in question was revealed by the Báb and has been associated with a variety of instructions for its recitation. It seems to us that his questions are answered in a letter dated 25 November 1999 written on behalf of the Universal House of Justice, as follows:

Concerning the phrase “Yá Alláhu’l-Mustagháth”, this is an invocation revealed by the Báb. He prescribed it for recitation by His followers in times of trouble and difficulty. Shoghi Effendi has translated the word “Mustagháth” as “He Who is invoked for help”. This phrase can be correctly transliterated in two ways, as set out below:

“Yá Iláha’l-Mustagháth”, which has been translated as “O Lord of the time of ‘Mustagháth’”

“Yá Alláhu’l-Mustagháth”, which has been translated as “O Thou God Who art invoked”

With regard to the number of times these words are to be repeated, the repetition of this invocation is not definitely fixed, and there is a great deal of flexibility concerning the repetition of this and other prayers. While the invocation is prescribed in the Writings of the Báb to be repeated 2098 times during occasions of great need, ‘Abdu’l-Bahá in one Tablet states that this verse is to be repeated 95 times and, in another Tablet, 81 times. Letters from the Guardian concerning this invocation, as well as other prayers, indicate that repetition is a matter of individual choice. In a postscript added in his own handwriting to a letter to an individual he stated:

There is no objection to saying “Yá Iláha’l-Mustagháth” any time you like and as often as you like.

In the Writings of the Báb, “Mustagháth” refers to Bahá’u’lláh, and “the time of ‘Mustagháth’” refers to the time of Bahá’u’lláh’s Dispensation. The Báb used these terms when He addressed the possible opposition of the divines and people of the Bayán to the coming Revelation. You may wish to study other references to “the mystery of the ‘Mustagháth’” in the Bahá’í Writings, such as The Kitáb-i-Íqán (Wilmette: Bahá’í Publishing Trust, 1983), pages 229–30, God Passes By (Wilmette: Bahá’í Publishing Trust, 1987), page 27; and The Dawn-Breakers: Nabíl’s Narrative of the Early Days of the Bahá’í Revelation (Wilmette: Bahá’í Publishing Trust, 1974), pages 304–305.

It may also be helpful to Mr. … to clarify the difference in the two transliterated forms of the invocation. The invocation reads as “Yá Alláhu’l-Mustagháth” if Alláh is a modified noun and Mustagháth is an adjective. However, if the words are used in the genitive case, then the phrase reads as “Yá Iláha’l-Mustagháth”. 
(From Memorandum from the Research Department of the Universal House of Justice to the Universal House of Justice, 28 December 2001; online Baha’i Library)

Additional Notes:

The meaning of Mustaghath is: 'He Who is invoked for help.' (From a letter written on behalf of Shoghi Effendi to an individual believer, June 14, 1947; (Compilation: ‘Lights of Guidance’)

Mustaghath: "He who is invoked"; the numerical value of which has been assigned by the Báb as the limit of the time fixed for the advent of the promised Manifestation. (The Glossary section of ‘The Dawn-Breakers’)

Shoghi Effendi also had the burden of the believers' personal griefs. Florence [Marzieh Gail’s mother] once asked him for a very powerful prayer, and he answered, 'What could be better than Ya Allahu'l-Mustaghath?' This is rendered 'O God, the One Who is invoked' -- its implication being, Who is called upon in times of extreme distress and peril. (Marzieh Gail, ‘Arches of the Years’)

The Master spoke at length about the withdrawal of the Blessed Beauty from Baghdad. He told of the prayers of the friends of God who recited, 'Ya Allah-ul-Mustaghath', the receipt of the of the bequest Aqa Abu'l-Qasim-i- Hamadani, their eventual tracing of Bahá'u'lláh to the place in Sulaymaniyyih where He had taken abode, and then their dispatching a petition to the Blessed Beauty for His return. (Mahmud’s Diary)