The subsequent course of the campaign is only provided by Ibn A'tham and the sources drawn from his work. According to this account, Marwan penetrated further north, and attacked a Slavic people called Burtas, whose territory extended up to that of the Volga Bulgars and who were Khazar subjects, taking 20,000 families (or 40,000 persons, in other accounts) captive. The Khazar army, under the "tarkhan" (one of the highest dignitaries in Turkic states), shadowed the Arab advance from the opposite shore of the Volga. As the Khazars avoided battle, Marwan sent a detachment of 40,000 troops across the Volga under al-Kawthar ibn al-Aswad al-'Anbari, which surprised the Khazars in a swamp. In the ensuing battle, the Arabs killed 10,000 Khazars, including the "tarkhan", and took 7,000 captives. Thereupon the Khazar "khagan" himself is said to have requested peace. Marwan reportedly offered "Islam or the sword", whereupon the "khagan" agreed to convert to Islam. Two "faqihs" were sent to instruct him on the details of the faith—the prohibition on consuming wine, pork, and unclean meat are specifically mentioned. Marwan also took with him large numbers of Slav and Khazar captives, whom he resettled in the eastern Caucasus: some 20,000 Slavs were settled at Kakheti, according to al-Baladhuri, while the Khazars were resettled at al-Lakz. Soon after, the Slavs killed their appointed governor and fled north, but Marwan rode after them and killed them.