Sien is the first historically-confirmed ruler of the tribal confederation known as the Kingdom of Diaokhi. He reigned toward the end of the 12th century BCE, most likely shortly after the creation of the kingdom. He joined his domains to Nairi, a political-military alliance between South Caucasus states.
In 1112 BCE, he had to face the invasion of Assyrian King Tiglath-Pileser I, who had launched an attack on Nairi states. Sien gathered 60 tribes spanning from the northeast of Anatolia to Colchis and formed a large army. But he was rapidly defeated and the Assyrian ruler pursued him until the Black Sea shores before capturing him.
Tiglath-Pileser I deposed Sien and had him deported toward his capital Assur when the latter refused to submit as had the other rulers of Nairi. A captive Sien had to accept his status as a client king of Assyria, recognizing the supremacy of the deity Ashur, and promised his and his descendant's loyalty toward Assyrian kings. He was then allowed to return to Diaokhi but was made to pay an annual tribute of 1,200 horses and 2,000 cattle.