The heretics from Babel Tower were originally defenders, and the empty city provided them with the perfect cover. They only needed to hide the flesh and blood of the Deity in the buildings in advance to nourish the Specter Fiends' hives lying dormant in the cocoons. All they had to do next was to retreat honestly to their strongholds.
When the military attacked, they would naturally encounter a Specter Tide ambush. Their Assassins only needed to snipe from the shadows, giving them an absolutely advantageous situation.
Indeed, during the five days since Yuan Lie led his forces into the Land of the Departed, they had not been able to achieve any significant victory and had nearly exhausted all of their ammunition, food, and Life Energy.
However, when they released the Specter Tide all at once, it was time for an all-out counterattack. As long as they could get the butterfly into Sacred Mountain, their mission would be complete.