Complex sentence: Kuber's subjects, who called themselves "Sermesianoi", but were regarded as "Bulgars" by the Byzantines, preserved their Christian traditions, although their ancestors had been taken to the Avar Khaganate about 60 years before Kuber's appointment.
Split into simpler sentences: Kuber's subjects called themselves "Sermesianoi", but were regarded as "Bulgars" by the Byzantines. They preserved their Christian traditions, although their ancestors had been taken to the Avar Khaganate about 60 years before Kuber's appointment.