Complex sentence: To secure his claim to Raumarike, Halfdan first defeated and killed Sigtryg, the previous ruler, in battle; then defeated Sigtryg's brother and successor Eystein, in a series of battles.
Split into simpler sentences: To secure his claim to Raumarike, Halfdan first defeated and killed the previous ruler, Sigtryg in battle. He then defeated Sigtryg's brother and successor Eystein, in a series of battles.