The King of Kings was the absolute ruler of Makuran for centuries. Much like its rival, the Empire of Videssos, the Kingdom of Makuran saw periods of great strength and stability followed by periods of incredible instability, and the fate of the reigning King of Kings could follow suit.
This page lists the small number of King of Kings Harry Turtledove identified throughout the Videssos series.
In The Misplaced Legion, Rishtaspa is named as a Makuran King of Kings who was killed when the Avtokrator Laskaris sacked Mashiz 739 years before the arrival of the Roman Legion. Laskaris is also described as having personally taken the helmet from Rishtaspa's corpse, which became a trophy for the empire.
However, these actions are credited to Stavrakios in every other volume of the series, and Rishtaspa is never directly named again after this.
Nakhorgan was on the Makuraner throne when Krispos became Avtokrator of Videssos. Nakhorgan's price for continued peace was the return of Makuraner territory taken by Petronas in a recent war. As Videssos was embroiled in the Kubrat War to the east, Krispos had no choice but to agree.
Nakhorgan is probably the incumbent King of Kings referenced in Krispos Rising chapter VI, the first time the office is mentioned. He seems to be still on the throne at the end of Krispos of Videssos, judging by his posthumous reference in Krispos the Emperor.
Rubyab, son of Nakhorgan, reigned concurrently with Krispos, Avtokrator of Videssos. Not wanting a head-on war with Videssos, Rubyab sponsored the Thanasioi rebellion within Videssos' borders to weaken Makuran's rival.
Rubyab is the incumbent King of Kings throughout Krispos the Emperor.
Varahran was the last King of Kings of Makuran. A few decades before the arrival of Marcus Aemilius Scaurus and the misplaced Legion, Avshar summoned a demon to slay Varahran while he slept. The Yezda conquered Makuran and renamed it Yezd in short order.