Complex sentence: However, when the Flood intelligence known as Gravemind managed to corrupt their war-coordinating AI, Mendicant Bias, thus giving the Flood the edge they needed to win, they had no choice but to activate the Halo network.
Split into simpler sentences: However, when the Flood intelligence known as Gravemind managed to corrupt their war-coordinating AI, Mendicant Bias, the Flood gained the edge they needed to win. The Forerunners had no choice but to activate the Halo network.