Piyashshili was a Hittite prince, and a son of King Suppiluliuma I. After Suppiluliuma I concluded a treaty with Shattiwazza, son of King Tushratta of Hanilgalbat, and married one of his daughters to him, Piyashshili led a Hittite army that put Shattiwazza on the throne of Hanilgalbat. According to Hittite sources, Piyashshili and Shattiwazza crossed the Euphrates at Carchemish, then marched against Irridu, already in Hurrian territory. After having reduced Irridu and Harran, they continued east towards Washshukanni and perhaps conquered the capital Taite as well. After Shattiwazza had been made a vassal ruler of Hanilgalbat, Piyashshili was given the territory of Ashtata (with the cities of Ekalte, Ahuna and Terqa) and Carchemish, formerly belonging to Hanigalbat, by his father. "And all of the cities of the land of Carchemish, Murmurik, Shipri, Mazuwati and _urun – these fortified cities– I gave to my son." (Suppililiumas-Shattiwazza treaty). In fact, the whole former territory of Hanilgabat west of the Euphrates seems to have come under direct Hittite rule and was governed by Piyashshili.
When the Egyptians attacked Kadesh (Kinza), the town was besieged and taken by Suppililiumas. The people were deported and Piyashshili made governor. Piyashshili fell ill and died before year 9 of Mursili II. After his death, a rebellion broke out in Quadesh and Nuhashshe. After it had been quelled, Piyashshili's son was made king of Quadesh by his uncle Mursili II.
External link
Text of the Suppiluliuma-Shattiwazza treaty (http://www.geocities.com/farfarer2001/hittite_letters/suppiluliuma_shattiwaza_treaty.htm)
After the conclusion of the Suppiluliuma-Shattiwaza treaty, Piyashshili, a son of Suppiluliuma, led a Hittite army into Mitanni.
Piyashshili and Shattiwaza marched on Washukanni, and the cities of Harran and Pakarripa seem to have surrendered to them.
Shattiwaza became king of Mitanni, but after Suppililiuma had taken Carchemish and the land west of the Euphrates, that were governed by his son Piyashshili, Mitanni was restricted to the Khabur river and Balikh River valleys, and became more and more dependent on their allies in Hatti.
After Shattiwazza had been made a vassal ruler of Hanilgalbat, Suppiluliumas gave to Piyashshili the Akkadian name Sarri-Kusuh and the territory of Ashtata (with the cities of Ekalte, Ahuna and Terqa) and Carchemish, formerly belonging to Hanigalbat.
The people were deported and Suppililiumas made Piyashshili governor of that town as well.
Piyashshili fell ill and died before year 9 of Mursili II.