Complex sentence: Survivors of the wars-the strongest and fittest-remained on the planet and interbred, unknowingly coerced by the Monitors and forming the Forerunners, a species possessing traits of the other planets such as those of Green Martians and Saturnians.
Split into simpler sentences: Survivors of the wars-the strongest and fittest-remained on the planet and interbred, unknowingly guided to do so by the Monitors. The process produced the Forerunners, a species possessing traits of the beings from other planets such as Green Martians and Saturnians.