Cotys I succeeded Seuthes II in 383. The historian Michael Zahrnt described Cotys as "the right man to strengthen the run-down Odrysian realm, vigorous, and an artful diplomat [...]." Indeed, it was under him that the kingdom reached its greatest might and became a considerable political factor in the nascent Hellenistic world. He was also the only Odrysian king whose character was excessively discussed by ancient scholars, although primarily in a rather unfavourable way. While virtually nothing is known about the early years of his rule it is clear that he, together with his son-in-law Iphicrates, managed to conquer the domains of the deceased Hebryzelmis, thus uniting the Odrysian realm under his rule. In 375 he faced an invasion of the Triballi, who devastated the western parts of the realm while marching towards Abdera at the coast.