At the news of his brother's murder, Vistahm rose in open revolt. According to Dinawari, Vistahm sent a letter to Khosrow announcing his claim to the throne through his Parthian (Arsacid) heritage: "You are not worthier to rule than I am. Indeed, I am more deserving on account of my descent from Darius, son of Darius, who fought Alexander. You Sasanians deceitfully gained superiority over us Arsacids and usurped our right, and treated us with injustice. Your ancestor Sasan was no more than a shepherd." Vistahm's revolt, like Bahrams's shortly before, found support and spread quickly. Local magnates as well as the remnants of Bahram Chobin's armies flocked to him, especially after he married Bahram's sister Gordiya. Vistahm repelled several loyalist efforts to subdue him, and he soon held sway in the entire eastern and northern quadrants of the Iranian realm, a domain stretching from the Oxus river to the region of Ardabil in the west. He even campaigned in the east, where he subdued two Hephthalite princes of Transoxiana, Shaug and Pariowk. The date of Vistahm's uprising is uncertain. From his coinage, it is known that his rebellion lasted for seven years. The commonly accepted dates are ca. 590–596, but some scholars like J.D. Howard–Johnston and Parvaneh Pourshariati push its outbreak later, in 594/5, to coincide with the Armenian Vahewuni rebellion.