Complex sentence: In September 1345, after a long siege, Serres fell to Dushan, spurring the Serbian ruler, who by now controlled about half of the pre- 1341 Byzantine realm, to lay his own claim to the Byzantine throne.
Split into simpler sentences: In September 1345, after a long siege, Serres fell to Dushan. The Serbian ruler, who by now controlled about half of the pre- 1341 Byzantine realm, was spurred to lay his own claim to the Byzantine throne.