Complex sentence: Once dismounted, Pollux charges Sejanus, but the Prefect kills him easily, turning him into an undead minion whom Castor is forced to fight, whilst The Spartan and Electra battle Sejanus.
Split into simpler sentences: Once dismounted, Pollux charges Sejanus, but is killed. Sejanus then turns him into an undead minion whom Castor is forced to fight, whilst The Spartan and Electra battle Sejanus.