The Kidarite king Grumbates mentioned by Ammianus Marcellinus was a cause of much concern to the Persians. Between 353 AD and 358 CE, the Xionites under Grumbat attacked in the eastern frontiers of Shapur II's empire along with other nomad tribes. After a prolonged struggle they were forced to conclude a peace, and their king Grumbat accompanied Shapur II in the war against the Romans. Victories of the Xionites during their campaigns in the Eastern Caspian lands are described by Ammianus Marcellinus:
Grumbates Chionitarum rex novus aetate quidem media rugosisque membris sed mente quadam grandifica multisque victoriarum insignibus nobilis.
"Grumbates, the new king of the Xionites, while he was middle aged, and his limbs were wrinkled, he was endowed with a mind that acted grandly, and was famous for his many, significant victories."— Ammianus Marcellinus, 18.6.22.
He participated in the Siege of Amida in 359 CE.