Tumgan (also known as Turkshad, Turxanthos) was a shad (governor prince) of the Turkic Empire (also called Göktürk) in the late 6th century.
Initially Turkic and Byzantine Empires were allies against Sassanid Iran and Pannonian Avars.
[3] Following this accusation, Turks began to capture Byzantine possessions around north east Black Sea coasts and even Crimea.
Briefly, a part of Bosporan Kingdom in Crimea, a vassal of Byzantine Empire fell to Turks.
(About half a century later Byzantine Turkic relations recovered and Turks collaborated with Heraclius in the invasion of North Caucasus.)