Complex sentence: However, upon arriving, Loghain abandons the battlefield with his army, leaving King Cailan, Duncan, and their army to be overwhelmed and slain by the Darkspawn, who seize control of Ostagar and begin advancing into southern Ferelden.
Split into simpler sentences: However, upon arriving, Loghain abandons the battlefield with his army, leaving King Cailan and Duncan to be overwhelmed and slain. The Darkspawn massacre Cailan's army, seize control of Ostagar, and begin advancing into southern Ferelden.