Complex sentence: Internal strife and attacks by neighboring tribes caused the decline of the Khaganate in the early 9th century, and the Magyars were among their subject peoples who seceded, settling in the Pontic steppes to the north of the Black Sea.
Split into simpler sentences: Internal strife and attacks by neighboring tribes caused the decline of the Khaganate in the early 9th century. The Magyars were among the Khazars' subject peoples who seceded, settling in the Pontic steppes to the north of the Black Sea.