The Swedes were particularly active in Eastern Europe, where they were known as the "Rus'". They were engaged in extensive trade with the Byzantine Empire and the Abbasid Caliphate, launching raids on Constantinople and expeditions in the Caspian Sea. The Rus' are described in detail by the Arab traveller Ahmad ibn Fadlan, who described them as tall, blond and the most "perfect physical specimens" he had ever seen. In the 9th century, the Viking Rurik is believed to have founded the Rurik dynasty, which eventually developed into Kievan Rus'. The North Germanic elite of this state were known as the Rus'. In the 10th century, the Rus', in cooperation with surviving Crimean Goths, destroyed the Khazar Khaganate and emerged as the dominant power in Eastern Europe. By the 11th century, the Rus' had converted to Eastern Orthodoxy and were gradually merging with the local East Slavic population, becoming known as the Russians. The North Germanic diaspora in the area were thereafter called Varangians. Many of them served in the Varangian Guard, the personal bodyguard of the Byzantine emperors. Among the prominent Scandinavians who served in the Varangian Guard were Norwegian king Harald Hardrada.