Complex sentence: They led Akpa mercenaries into the Aro territory to assist their Igbo allies to victory but with the cost of Osim losing his life in the start of the 18th century.
Split into simpler sentences: They led Akpa mercenaries into the Aro territory to assist their Igbo allies to victory. However, this came at the cost of Osim losing his life at the start of the 18th century.