After the death of Kavad II, Heraclius sent Shahrbaraz a letter saying: 
Now that the Iranian king is dead, the throne and the kingdom has come to you.
I bestow it on you, and on your offspring after you.
If an army is necessary, I shall send to your assistance as many [troops] as you may need.

On 27 April 630 Shahrbaraz besieged Ctesiphon with a force of 6,000 men.
He was, however, unable to capture the city, and then made an alliance with Piruz Khosrow, the leader of the Parsig (Persian) faction, and the previous minister of the Empire during the reign of Ardashir's father, Kavad II.
He also made an alliance with Namdar Gushnasp, who had succeeded him as the spahbed of Nēmrōz in 626.
Shahrbaraz, with the aid of these two powerful figures, captured Ctesiphon, and executed Ardashir III along with many Sasanian nobles, including Ardashir's minister Mah-Adhur Gushnasp.
Shahrbaraz then became the new shah (king) of the Sasanian Empire, and killed Kardarigan, who opposed Shahrbaraz after his usurpation of the Sasanian throne.

Heraclius also acknowledged Shahrbaraz's Christian son Niketas, as his heir.
An Iranian Christian as the heir of the Sasanian Empire opened the chances of the Christianization of Iran.
After some time, Shahrbaraz had Shamta, the son of the former financial minister Yazdin, crucified on a church in Margha.
The reason of this execution was reportedly because the latter had insulted Shahrbaraz during the reign of Khosrow II.
Forty days later, 9 June 630, during a ceremony, Shahrbaraz was killed by a javelin thrown by Farrukh Hormizd, who then helped Boran, the daughter of Khosrow II, to ascend the throne.
After the death of Kavad II, Heraclius sent Shahrbaraz a letter saying: 
Now that the Iranian king is dead, the throne and the kingdom has come to you. I bestow it on you, and on your offspring after you. If an army is necessary, I shall send to your assistance as many [troops] as you may need.
On 27 April 630 Shahrbaraz besieged Ctesiphon with a force of 6,000 men. He was, however, unable to capture the city, and then made an alliance with Piruz Khosrow, the leader of the Parsig (Persian) faction, and the previous minister of the Empire during the reign of Ardashir's father, Kavad II. He also made an alliance with Namdar Gushnasp, who had succeeded him as the spahbed of Nēmrōz in 626. Shahrbaraz, with the aid of these two powerful figures, captured Ctesiphon, and executed Ardashir III along with many Sasanian nobles, including Ardashir's minister Mah-Adhur Gushnasp. Shahrbaraz then became the new shah (king) of the Sasanian Empire, and killed Kardarigan, who opposed Shahrbaraz after his usurpation of the Sasanian throne.
Heraclius also acknowledged Shahrbaraz's Christian son Niketas, as his heir. An Iranian Christian as the heir of the Sasanian Empire opened the chances of the Christianization of Iran. After some time, Shahrbaraz had Shamta, the son of the former financial minister Yazdin, crucified on a church in Margha. The reason of this execution was reportedly because the latter had insulted Shahrbaraz during the reign of Khosrow II. Forty days later, 9 June 630, during a ceremony, Shahrbaraz was killed by a javelin thrown by Farrukh Hormizd, who then helped Boran, the daughter of Khosrow II, to ascend the throne.
Shahrbaraz