Hadrian retained control over Osroene through the client king Parthamaspates, who had once served as Trajan's client king of Parthia; and around 121, Hadrian negotiated a peace treaty with the now-independent Parthia. Late in his reign (135), the Alani attacked Roman Cappadocia with the covert support of Pharasmanes, the king of Caucasian Iberia. The attack was repulsed by Hadrian's governor, the historian Arrian, who subsequently installed a Roman "adviser" in Iberia. Arrian kept Hadrian well-informed on matters related to the Black Sea and the Caucasus. Between 131 and 132, he sent Hadrian a lengthy letter ("Periplus of the Euxine") on a maritime trip around the Black Sea that was intended to offer relevant information in case a Roman intervention was needed.