Shahr-bánú (literally “The Fairest Lady of the Realm, shahr means dominion/realm and bánú, “fairest, brightest lady”), is said to be the daughter of Yazdgerd III (632-51 CE), the last Zoroastrian Sassanid Emperor. According to the beliefs of the Shiʿites, (also of a substantial number of Sunnis,) she became the wife of the third Imam, Hussain ibn ʿAli, and the mother of the fourth Imam, Zayn ul-ʿĀbedin. Consequently, the lineage of Imams, from the fourth to twelfth and final, would be her progeny.
In his al-Kāmel, the philologist Mobarrad (died 900 CE) seems to have been one of the very first to state that the mother of Imam ʿAli Zayn al-ʿĀbedin, was the daughter of Emperor Yazdgerd. He strongly emphasizes the nobility of the woman and, in general, the grandeur of the Ancient Persians (Mobarrad II, pp. 645-66).
Yaʿqubi (904 CE), Ḥasan b. Musā Nowbaḵti and Saʿd b. ʿAbd-Allāh (both died circa. 912-13 CE) are among the first Shiʿites to allude in passing to the fact that the mother of Imam Zayn al-ʿĀbedin was the daughter of the last Zoroastrian Sassanid Emperor (Yaʿqubi II, pp. 246-47 and 303; Nowbaḵti, p. 53; Ašʿari, p. 70).
Also, Ṣaffār Qomi (died 902-903 CE) delivers a long and detailed version of the account, containing a Hadith or saying attributed to the fifth Imam Moḥammad Bāqer: Accordingly, under the second caliph ‘Omar (reign 634-44 CE), the noble daughter of the last Zoroastrian Sassanid Emperor is brought as a captive to Medina. Light radiating from the fair face of the Zoroastrian princess illuminated the Prophet of Islam’s mosque, where the caliph presides. A prayer in the sacred Avestan language by the Zoroastrian Princess provoked Omar’s wrath.
ʿAli intervened in favor of the young Zoroastrian princess and made it clear to ‘Omar that events unfolding are beyond his understanding and that he should step aside. ʿAli then authorized the Zoroastrian Princess, with whom he spoke in Persian, to freely choose her husband. She chose Ḥussain to whom ʿAli announced the good news that the young woman will be the mother of his child, i.e. the next Imam (Ṣaffār, p. 335, no. 8).
In the Eṯbāt al-waṣiya, attributed to Masʿudi (died 956-57 CE), the story takes place under the caliphate of ‘Omar and in this case two daughters of Emperor Yazdgerd are given in marriage, with ʿAli’s consent to his sons (Masʿudi, p. 170).
In Shaykh Mofid’s (died 1022 CE) account, under ʿAli’s caliphate, the elder daughter of the Persian Emperor, here named shāh-e zanān (lit.: “Ruler of all women” cCompare with the title of Fāṭema, sayyedat al-nesāʾ) marries Ḥussain, while a second unnamed daughter is given in marriage to the son of Abu Bakr, Moḥammad (Mofid, pp. 137-38).
All the Western specialists of Sassanid history, from Darmesteter to Christensen, to Nöldeke or Spuler, state that the royal family had been evacuated from the capital Ctesiphon well before the Arab invasion. Their account seem to agree with a number of islamic sources such as Maškur, II, pp. 1288 ff. and 1344 ff.; Ḥaṣuri.
However, other Islamic sources report the capture and reduction to slavery of a kin (not necessarily the daughter) of Emperor Yazdgerd III. The young Princess was supposedly captured in northern Khorasan, and is said to have been sent to the arab governor, who in turn have send her to the caliph.
Sources from the China of the T’ang dynasty (r. 618-907) regarding the Arab conquest of Persia, remain silent about an eventual captivity of one of the members of the family of Emperor Yazdgerd III (Marquart, pp. 68-69; Chavannes, pp. 171-73; Hoyland, pp. 243 ff.).
However, it is very likely that a Zoroastrian Sassanid Princess was captured, reduced to slavery and given in marriage to a noble-man or Imam of the arab conquerors. The fact that both sunni and shia accounts report a very similar story makes it much more plausible.
Interestingly, both sunni and shia accounts emphasize the ahl-e-kitab status (people of the book status) of the young Princess meaning Shahr-bánú has always stayed a Zoroastrian.
In the literary tradition, Shahr-bánú passes away by drowning in the Euphrates, after having witnessed the massacre of her family at Karbalā.
Popular belief decidedly preferred otherwise; the most recurrent version of the oral legend of the daughter of Emperor Yazdgerd III, here called Bibi (respectable Lady or grandmother) Šahrbānu: after the day of ʿĀšurāʾ Bibi Shahr-bánú is able to escape, as had predicted her husband, with Ḏu ʾl-Janāḥ, the horse of Imam Hussain.
Pursued by her terrifying enemies, she reaches up to the mountain at Ray, south of the mighty Alborz Mountains. Exhausted, and at the limits of her strength, all alone, she invokes God to be delivered from her assailants.
At this point, the mountain miraculously opens and offers refuge to the princess. The location became a sanctuary to the princess, a pilgrimage site to remain so until today (Šahidi, pp. 186 ff.).
An almost identical story is found to be at the source of the Zoroastrian sanctuary of Bānu Pārs (Lady of Persia), northwest of the plain of Yazd (Sorušiān, p. 204). More generally, themes such as the escape of Zoroastrian Persian nobles (often members of the royal family) from the arabs and their miraculous rescue by God thanks to elements of nature are frequently appear in legends of Zoroastrian sanctuaries in central mountain of Iran Persia (Sorušiān, pp. 205-11; Strack, I, pp. 119 and 227-28).
The above article and all the citations therein are taken entirely from Encyclopedia Iranica, my personal interjections and revisions are added in some places