The Parthian "dark age", a confused period that lasted some three decades, began in 91 BCE after the succesful reign of Mithradates II, who left the throne to his son Gotarzes I while there was a rebel, Sinatruces. Gotarzes managed to sideline his rival but died soon after, in 87. He was succeeded by a relative named Mithradates III who lost a war against the Armenian king Tigranes II and lost his throne to a new pretender: Orodes I (spring 80 BCE).
After some five years, in 75 BCE, Orodes' coinage comes to an end and new coins were minted by an otherwise king, provisionally called Arsaces XVI by scholars. This crisis may have been triggered by the return of Sinatruces, the former rebel king, who may have had support from the nomadic Sacae in the northwest. Alternatively, the downfall of Orodes may have offered an opportunity to Sincatruces.
However this may be, Arsaces XVI gained control of the greater part of the Parthian Empire and ruled quite successfully, although Sinatruces remained active in the northeast and was succeeded as rebel king by his son Phraates III. It seems that this Phraates seized the western part of the kingdom from Arsaces XVI, but was unable to decisively defeat him, because Arsaces XVI still minted coins in 62.
In any case, after 62/61, Phraates was the uncontested sole ruler of the Parthian Empire until 58/57 BCE, which marks the end of the "dark period" in Parthian history.
His reign also marks Parthian intervention in the far west, where the Roman Republic waged war against Mithridates VI Eupator of Pontus and his ally Tigranes II of Armenia (the Third Mithridatic War, 73-63). The Roman general Lucullus had sent envoys to the Parthian court, which had been received by Arsaces XVI, but the negotiations were completed by Phraates III, who agreed with Lucullus' successor in command, Pompey the Great, to invade Armenia while the Romans were attacking Pontus (65 BCE). Together, they brought the war to a succesful conclusion.
Phraates III, the man who had reunited the Parthian Empire, was assassinated in 58/57 and was succeeded by his sons Mithradates IV and Orodes II.
The chronology of the Arsacid kings of the Parthian Empire is less well-understood than, for example, the sequence of Seleucid and Ptolemaic kings or the emperors of Rome. This information is based on the researches by G.R.F. Assar, as published in "Iran under the Arsakids, 247 BC – AD 224/227" in: Numismatic Art of Persia (2011).