Complex sentence: With no other enemy left to fight, they used their battle machines in epic duels to the death-this was considered their golden age, the era of Heroic Combat, but it was leading to the extinction of their race.
Split into simpler sentences: With no other enemy left to fight, they used their battle machines in epic duels to the death. This was considered their golden age, the era of "Heroic Combat", but it led to the extinction of their race.