Question: Ashtat Yeztayar was an Iranian military officer under what last great king of the Sasanian Empire, reigning from 590 to 628?

Context paragraphs:
Senitam Khusro: Senitam Khusro, was an Iranian military officer, who served as the commander of Armenia from 605/6 to 606/7 under the Sasanian king Khosrow II (r. 590–628).

Farrukhzad: Farrukhzad (Middle Persian: "Farrūkhzādag" ; New Persian: فرخزاد ), was an Iranian aristocrat from the House of Ispahbudhan and the founder of the Bavand dynasty, ruling from 651 to 665.  Originally a powerful servant of the Sasanian king Khosrow II (r. 590-628), he, along with several other powerful aristocrats made a conspiracy against the latter and ended his tyrannical rule.  They thereafter put Khosrow's son Kavadh II (r. 628) on the throne, whose rule lasted only a few months, before he was killed by a plague, being succeeded by his son Ardashir III (r. 628-629), who was only after one year murdered by the rebellious former Sasanian army chief ("spahbed") Shahrbaraz, who usurped the throne.

Military of the Sasanian Empire: The Sasanian army (Middle Persian "Spâh-ī Sāsānīg") was the primary military body of the Sasanian armed forces, serving alongside the Sasanian navy.  The birth of the army dates back to the rise of Ardashir I (r. 224–241), the founder of the Sasanian Empire, to the throne.  Ardashir aimed at the revival of the Persian Empire, and to further this aim, he reformed the military by forming a standing army which was under his personal command and whose officers were separate from satraps, local princes and nobility.  He restored the Achaemenid military organizations, retained the Parthian cavalry model, and employed new types of armour and siege warfare techniques.  This was the beginning for a military system which served him and his successors for over 400 years, during which the Sasanian Empire was, along with the Roman Empire and later the East Roman Empire, one of the two superpowers of Late Antiquity in Western Eurasia.  The Sasanian army protected "Eranshahr" ("the realm of Iran") from the East against the incursions of central Asiatic nomads like the Hephthalites and Turks, while in the west it was engaged in a recurrent struggle against the Roman Empire.

Shahrbaraz: Shahrbaraz or Shahrvaraz (Middle Persian: 𐭧𐭱𐭨𐭥𐭥𐭥𐭰, "Šahrwarāz"; <a href="Persian%20language">Persianهربراز‎ ‎ , "Šahrbarāz"; died 9 June 630) was king of the <a href="Sasanian%20Empire">Sasanian Empire</a> from 27 April 630 to 9 June 630.  He usurped the throne from Ardashir III, and was killed by Sasanian nobles after forty days.  Before usurping the Sasanian throne he was a general ("spahbed") under Khosrow II (590–628).  He is furthermore noted for his important role during the climactic Byzantine–Sasanian War of 602–628, and the events that followed afterwards.  His name Shahrbaraz is actually an honorific title, and means "the Boar of the Empire", attesting to his dexterity in military command and his warlike personality, as the boar was the animal associated with the Zoroastrian Izad Vahram, the epitome of victory.

Khosrow II: Khosrow II (Chosroes II in classical sources; Middle Persian: "Husrō(y)"), entitled "Aparvēz" ("The Victorious"), also Khusraw Parvēz (New Persian: خسرو پرویز ), was the last great king of the Sasanian Empire, reigning from 590 to 628.

Shahriyar (son of Khosrow II): Shahriyar (also spelled Shahryar) was a Sasanian prince—he was the son of Khosrow II (r. 590-628) and his Christian queen Shirin.  In 628, a son of Khosrow II, Kavadh II, staged a "coup d'état" against his father, and thereafter had all his brothers and half-brothers executed.  Shahriyar was survived by his son Yazdegerd III, who would later rule the Sasanian Empire from 632 to 651.

Ashtat Yeztayar: Ashtat Yeztayar was an Iranian military officer under the Sasanian king Khosrow II (r. 590–628).

Kavadh II: Shērōē (also spelled Shīrūya, شیرویه in New Persian), better known by his dynastic name of Kavadh II (Middle Persian: kwʾt' "Kawād"; New Persian: قباد‎ ‎ "Qobād" or "Qabād"), was king of the Sasanian Empire briefly in 628.  He was the son of Khosrau II (590–628).  He became king after orchestrating a coup d'état against his father.

Boran: Boran (Middle Persian: ; Persian: پوراندخت (Purandokht) 590–628 or 631) was empress regnant of the Sasanian Empire.  Boran was the daughter of emperor Khosrow II.  She was the first and one of only two women to rule the Sasanian Empire; the other was her sister and successor, Azarmidokht.  Various authors place her reign between one year and four months to two years.

Sasanian architecture: Sasanian architecture refers to the Persian architectural style that reached a peak in its development during the Sasanian era.  In many ways the Sasanian Empire period (224-651 CE) witnessed the highest achievement of Persian civilization, and constituted the last great pre-Islamic Persian Empire before the Muslim conquest.  In fact part of Sasanian architecture was adopted by Muslims and became part of Islamic architecture.

Supporting paragraphs: Ashtat Yeztayar Khosrow II
Answer: Khosrow II