Question: What is the name of the geographical region that early Islamic usage often regarded everywhere east of western Persia and where Vistaham helped Khosrow regain his throne after the rebellion of Bahram Chobin?  

Context paragraphs:
The Stolen Throne: The Stolen Throne is a fantasy novel written by Harry Turtledove and set in the Videssos universe.  It is the first book in the "Time of Troubles" tetralogy.  The events depicted are strongly based on the historical interaction of Sassanid Persia and Byzantium in the 6th and 7th century.  The first book depicts the rise of Sharbaraz (the analog to Khosrau II) to overcome the usurper Smerdis (Bahram Chobin) to become the King of Kings of Makuran (Persia) with the help of the Videssian Emperor Likinios (Maurice).

Zoarab: Zoarab was king of the Daylamites in the late 6th-century.  He is first mentioned in 590, when he together with Sarames the Younger, betrayed the Sasanian king Hormizd IV (r. 579–590) by murdering his general Pherochanes.  Zoarab then joined the rebellion of Bahram Chobin, while Sarames joined a group of dissatisfied nobles led by Vistahm and Vinduyih.

Shirin: Shirin (?  – 628 AD) (Persian: شيرين‎ ‎ ) was a wife of the Sassanid Persian Shahanshah (king of kings), Khosrow Parviz.  In the revolution after the death of Khosrow's father Hormizd IV, the General Bahram Chobin took power over the Persian empire.  Shirin fled with Khosrow to Syria, where they lived under the protection of Byzantine emperor Maurice.  In 591, Khosrow returned to Persia to take control of the empire and Shirin was made queen.  She used her new influence to support the Christian minority in Iran, but the political situation demanded that she do so discreetly.  Initially she belonged to the Church of the East, the so-called Nestorians, but later she joined the miaphysite church of Antioch, now known as the Syriac Orthodox Church.  After conquering Jerusalem in 614, amidst the Byzantine–Sasanian War of 602–628, the Persians captured the True Cross of Jesus and brought it to their capital Ctesiphon, where Shirin took the cross in her palace.

Vistahm: Bistam or Vistahm (also transliterated Wistaxm, Persian: بیستام‎ ‎ ), was a Parthian dynast of the Ispahbudhan house, and maternal uncle of the Sasanian "shah" of Persia, Khosrow II (reigned 591–628).  Vistahm helped Khosrow regain his throne after the rebellion of Bahram Chobin, but later led a revolt himself, which encompassed the entire Iranian East before being suppressed.

Vinduyih: Vinduyih (Middle Persian: "Windôē") or Bendoy (New Persian) was a Sasanian nobleman from the Ispahbudhan family.  His sister was the mother Khosrau II, thus making Vinduyih the uncle of Khosrau.  Vinduyih and Vistahm played an important role in restoring the throne for Khosrau II from Bahram Chobin.  He was later deposed in Ctesiphon by the orders of Khosrau II.

Mardansina: Mardansina (also spelled Mardan Sina) was a Parthian nobleman from the House of Mihran—he was the son of Bahram Gushnasp and thus the brother of the prominent Sasanian military leader Bahram Chobin, who managed to overthrow the Sasanian king himself briefly in 590–591, but was eventually defeated and killed.  Mardansina thereafter became the new leader of the rebel movement of Bahram Chobin, and later took part in the rebellion of Vistahm (591–596 or 594/5–600).

Narses (general under Maurice): Narses was a Byzantine general of Armenian ancestry active during the reigns of the emperors Maurice and Phocas in the late sixth and early seventh centuries.  He commanded the army in Mesopotamia under Maurice.  Together with Khosrow II, he fought against the Sasanian usurper Bahram Chobin.  When Phocas overthrew Maurice and seized the throne, Narses refused to recognize the usurper.  Besieged by Phocas' troops in the city of Edessa, Narses called for Khosrau to aid him and was rescued by the Persian forces.  He attempted to salvage the situation with a diplomatic mission but was burned alive in Constantinople by Phocas' government after having been promised safety.

Sarames the Elder: Sarames the Elder was an Iranian officer of Median origin, who served in high offices under the Sasanian king Hormizd IV (r. 579–590), possibly as the governor of a province.  When Bahram Chobin rebelled against Hormizd IV, Sarames was sent to suppress his rebellion, but was defeated and captured by the latter, who had him trampled to death by elephants.

Greater Khorasan: Khorasan (Middle Persian: , Persian: خراسان‎ ‎ "Xorâsân"    ) is a historical region lying in the northeast of Persia.  However, in pre-Islamic and early Islamic times, the term "Khurassan" frequently had a much wider denotation, covering also parts of Central Asia and Afghanistan; early Islamic usage often regarded everywhere east of western Persia, so-called Jibal or what was subsequently termed 'Irak 'Adjami, as being included in a vast and ill-defined region of Khurasan, which might even extend to the Indus Valley and Sind.

Taqlid: Taqlid or taqleed (Arabic تَقْليد "taqlīd") is an Islamic terminology denoting the conformity of one person to the teaching of another.  The person who performs taqlid is termed "muqallid".  The definite meaning of the term varies depending on context and age.  Classical usage of the term differs between Sunni Islam and Shia Islam.  Sunni Islamic usage designates the unjustified conformity of one person to the teaching of another, apart from justified conformity of layperson to the teaching of "mujtahid" (a person who is qualified for independent reasoning).  Shia Islamic usage designates the general conformity of non-mujtahid to the teaching of mujitahid, and there is no negative connotation.  In contemporary usage, especially in the context of Islamic reformism, it is often shed in a negative light, and translated as "blind imitation".  This refers to the perceived stagnation of independent intellectual effort ("ijtihad") and uncritical imitation of traditional religious interpretation by the religious establishment in general.

Supporting paragraphs: Vistahm Greater Khorasan
Answer: Khorasan