Short answers, pulled from the story.
The Sasanian Empire was founded in 224 AD by Ardashir I, who defeated and killed the Parthian king Artabanus IV at the Battle of Hormozgan. Ardashir was crowned at Ctesiphon that same year, taking the title shahanshah, or King of Kings.
The official name was Eranshahr, meaning the Empire of the Iranians. The title is first attested in the trilingual Great Inscription of Shapur I, in which the king states "I am the lord of the Empire of the Iranians."
Decades of war with Byzantium left the Sasanian state financially exhausted and politically fractured. In 637 AD a Muslim army under Caliph Umar ibn al-Khattab defeated Persian general Rostam Farrokhzad at al-Qadisiyyah and captured Ctesiphon. The last emperor, Yazdegerd III, was assassinated by a miller in Merv in late 651.
Zoroastrianism was the official state religion, and the Sasanian rulers assumed guardianship of the sacred fire as a symbol of that role. The collection of the Avesta, Zoroastrianism's sacred texts, was completed during the reign of Shapur II.
Chinese documents record sixteen Sasanian embassies to China between 455 and 555 AD. Sasanian kings sent musicians and dancers to the Chinese imperial courts at Luoyang and Chang'an, and large numbers of Sasanian coins found in southern China confirm active maritime trade. After the Arab conquest, Peroz III, son of the last Sasanian emperor, was appointed Governor of Persia by the Chinese court in 661.
Khosrow I, also known as Anushirvan, ruled from 531 to 579 AD. He reformed taxation, created a state-salaried cavalry class called dehqans to replace feudal military levies, rebuilt infrastructure, and promoted cultural exchange with India, including commissioning the Pahlavi translation of the Indian Panchatantra. The 10th-century historian al-Masudi praised the Sasanian kings specifically for their well-ordered administration, and Khosrow I is remembered as the peak of that tradition.