on multiple occasions but never being able to hold on to them. Frequent civil wars between Parthian contenders to the throne proved more dangerous to the Empire's stability than foreign invasion, and Parthian power evaporated when Ardashir I, ruler of Istakhr in Persis, revolted against the Arsacids and killed their last ruler, Artabanus IV, in 224 AD. Ardashir established the Sasanian Empire, which ruled Iran and much of the Near East until the Muslim conquests of the 7th century AD, although the Arsacid dynasty lived on through branches of the family that ruled Armenia, Iberia, and Albania in the Caucasus. Native Parthian sources, written in Parthian, Greek and other languages, are scarce when compared to Sasanian and even earlier Achaemenid sources. Aside from scattered cuneiform tablets, fragmentary ostraca, rock inscriptions, drachma coins, and the chance survival of some parchment documents, much of Parthian history is only known through external sources. These include mainly Greek and Roman histories, but also Chinese histories, prompted by the Han Chinese desire to form alliances against the Xiongnu. Parthian artwork is viewed by historians as a valid source for understanding aspects of society and culture that are otherwise absent in textual sources. History Origins and establishment Before Arsaces I founded the Arsacid Dynasty, he was chieftain of the Parni, an ancient Central Asian tribe of Iranian peoples and one of several nomadic tribes within the confederation of the Dahae. The Parni most likely spoke an eastern Iranian language, in contrast to the northwestern Iranian language spoken at the time in Parthia. The latter was a northeastern province, first under the Achaemenid Empire, and then the Seleucid Empire. After conquering the region, the Parni adopted Parthian as the official court language, speaking it alongside Middle Persian, Aramaic, Greek, Babylonian, Sogdian and other languages in the multilingual territories they would conquer. Why the Arsacid court retroactively chose 247 BC as the first year of the Arsacid era is uncertain. A.D.H. Bivar concludes that this was the year the Seleucids lost control of Parthia to Andragoras, the appointed satrap who rebelled against them. Hence, Arsaces I "backdated his regnal years" to the moment when Seleucid control over Parthia ceased. However, Vesta Sarkhosh Curtis asserts that this was simply the year Arsaces was made chief of the Parni tribe. Homa Katouzian and Gene Ralph Garthwaite claim it was the year Arsaces conquered Parthia and expelled the Seleucid authorities, yet