In the wake of the dynastic squabbling and Tigranes' invasion, the Parthian state was in poor shape, even though Phraates III stood uncontested as King by 667 AUC (87 BC). It had taken him two long years to secure the throne, during which two of the Parthian capitals had been occupied by enemy forces. The continued raids by Scythian nomads did not improve the situation. The Arsacid dynasty stood on its last legs.
Phraates attempted, to the best of his ability, to stabilize his kingdom. However, the state was incredibly decentralized, with many minor lords ruling with virtual independence. Attempting to assert royal authority only invited revolt, and Phraates spent much of his time campaigning, always on the move, making sure that all the nobility knew that he was in charge.
In an effort to improve the standing of the Arsacids, Phraates led a campaign against the newly independent Persians in 672 AUC (82 BC). However, the Persian king ruled over a much more stable state than did Phraates, and he was also able to secure the allegiance of Elymais, the state to the west of Persia. The campaign quickly devolved into a quagmire instead of the quick prestigious victory Phraates had hoped for.
Before he could extricate himself from Persia, Phraates was faced with yet another challenge. The constant raiding by the Scythians was overshadowed by a new threat. Maues, an Indo-Scythian king, invaded their realm. It is unknown precisely which tribe he hailed from, but he had already been engaged in subjugating various Indo-Greek cities in Arachosia (Arghandab, Afganistan) to his rule. However, the destabilized Parthian kingdom was a much more tempting target at this time, particularly as the remaining Indo-Greek kingdoms were willing to support his campaign if he left them alone, effectively bribing him to invade westward instead of eastward.
Maues' army, like many Scythian armies, was largely cavalry-based. However, two key difference stood out. First, Maues' relations with the Greek cities provided him with a corps of experts familiar with siegecraft and other battlefield engineering. Second, his army was one of the first recorded in the west to have made use of stirrups, though they were of the Indian style, toe stirrups, which were of less use in cooler climates. Though it is doubtful that he was of their tribe, Maues had significant support among the Xanthii tribe, who formed a large contingent of his army. Due to this, sources in the west would regard the kingdom over which he ruled to be the Xanthian Kingdom.
Maues' army swept into Parthia, defeating the initial forces sent against it. Phraates was forced to concede to a status quo peace with Persia to turn and face the invasion. By the time he was able to meet Maues in battle, the Xanthians had already reached the Parthian capital at Hecatompylos and was in the process of besieging the city as Phraates' forces reached his. Withdrawing from the siege, Maues was able to lure Phraates into battle on terrain that gave the Xanthians the advantage, defeating Phraates, who died in the battle.
However, Phraates son, Orodes, was able to quickly secure the Arsacid throne and muster up a new army to fight the Xanthians. Equal to Maues in battle, Orodes was able to halt the onslaught of the Xanthian invasion for some time. However, Maues was able to reach Hecatompylos again in 674 AUC (80 BC) and took the city by treachery. Orodes attempted to siege the city while Maues' main army was away. However, Maues had marched off as a ruse, and soon returned to trap Orodes' army between the city and his army, crushing the force. The remainder of the Parthian army fled west. Maues fought against Orodes once more, at an unknown site some distance from Rhagae and defeated him again, capturing the young king in the battle.
Maues allowed Orodes to retain his crown, though as a vassal of Maues, and ruling over a greatly diminished kingdom in the original Parthian heartland. Maues married Orodes' sister and took the title of King of Kings of Iran, before marching against Persia, Elymais, and Characene in 675 AUC (79 BC), quickly subduing those states. His conquering spirit satisfied, Maues set about consolidating his empire, which stretched from Mesopotamia in the west to Arachosia in the east. Where the Arsacid Dynasty of Parthia once stood, the Mauid Dynasty of Xanthia had taken its place.
Maues was, much like the Arsacid kings he replaced, an admirer of Greek culture, having risen to power amongst the Greek cities of northern India, with whom he formed excellent ties after conquering Iran. A Buddhist, he established a tolerant respect for other religions, issuing coins with varying religious iconography in different regions of his empire. He worked to centralize the core regions of his empire, while allowing some autonomy to the outlying vassals. He also accepted many envoys from distant peoples, such as Rome and Seres.
Maues was an interesting character I discovered while searching for some Scythian barbarian to conquer the Parthians, so I had to include him in the fun. I had been researching the Xanthii, whose name met my criteria for invading Parthia: having a cool and easily latinized name. Check. Then, while looking for an actual king, I came across Maues. Unfortunately, nobody knows his nationality, and the Xanthii aren't even likely candidates, so I've had to rely on the tendency of historians to not let the truth get in the way of a good story for the names to come together.
His empire is the largest departure from our history to date. Whereas everything else that happened so far (apart from coffee) happened in some similar fashion in our history, Parthia originally lasted roughly three more centuries before finally falling to the Sassanid Persians. Having moved their collapse up so far will lead to all sorts of interesting consequences, in the east and the west. I'm also hand-waving his use of stirrups into the picture. Don't expect to see full stirrups in any updates soon, but contact with the Xanthian Empire will introduce the concept and lead, eventually, to someone developing them earlier than our history. Look forward to exciting happenings.