Complex sentence: Later that year as Sancho was mopping up the last of the resistance, besieging his sister Urraca at Zamora in October, he was assassinated, opening the way for Alfonso to return to claim Sancho's crown.
Split into simpler sentences: Later that year as Sancho was mopping up the last of the resistance, besieging his sister Urraca at Zamora in October, he was assassinated. This opened the way for Alfonso to return to claim Sancho's crown.