Parni: nomad tribe from Central Asia, ancestors of the Parthians.
The Central-Asian steppe has been the home of nomad tribes for centuries. Being nomads, they roamed across the plains, incidentally attacking the urbanized countries to the south, east and west. They are known under many names (more).
One of these tribes was that of the Parni. They are unknown before the third century BCE, the country where they lived, along the river Syrdar'ya (Jaxartes), was occupied by the tribe that the Persians knew as the Dahâ (litterally "robbers"). It is likely that this tribe disintegrated after the fall of the Achaemenid empire; the new rulers, the kings of the Seleucid dynasty, were never able to fully control the country of what is now Uzbekistan and Turkmenistan, Sogdia. Perhaps the Parni came into being in this period. In any case, nomads started to move to the south, to the countries known as Bactria, Aria, and Parthia.
The Seleucid king Antiochus I Soter (280-261) was the first to take measures. It is certain that he refounded a city in Margiana; up till then, it had been known as Alexandria (because it was founded by Alexander the Great or his general Craterus in 328), but from now on, it was to be called Antiochia. This military settlement was intended to guard Iran against incursions from nomad tribes, such as the Parni.
It was no sufficient, however. In 245, the satrap of Parthia, a man named Andragoras, revolted from the young Seleucid king Seleucus II Callinicus, who had just succeeded to the throne. In the confusion, the Parni attacked and seized the northern part of Parthia, a district known as Astavene, probably in 238. About 235, a Parnian prince with the name Tiridates ventured further south and seized the rest of Parthia. An counter-offensive by king Seleucus ended in disaster, and Hyrcania was also subdued by the Parni.
From now on, the Parni were known as Parthians. In the years that followed, their kings recognized the Seleucid king as their superiors, but under Mithradates I (171-138 BCE) they conquered Media, Babylonia, and Elam. The Parthian empire was to last until 224 CE, when it was succeeded by the Sasanian empire.