Complex sentence: In 279, Teiranes died and Theothorses succeeded him as the sole rler of the Bosporan Kingdom, reigning from 278 until his death around 308/309.
Split into simpler sentences: In 279, Teiranes died and Theothorses succeeded him and became the sole King of the Bosporan. Theothorses reigned as Bosporan King from 278 until his death around 308/309.