In order to quickly subdue the enemy, the Ground Spiders fumigate their enemy’s nest with poisonous gas.
There have also been cases where indigenous colonies will flood their enemies by drawing water from the rivers. But the objective of war is to capture and enslave the opposing colony, so tactics that would cause mass deaths are unsuitable. On the other hand, the wars on the mainland are mostly centered around acquiring and protecting limited resources, so killing the enemy might be more efficient in their case.
The gas that they used remains unidentified to this day. The remains of the gas dispersal equipment we found only tells us that the Ground Spiders built a furnace out of stones and mud upwind from the Robber Flies.
I guessed that the rotten-egg smell came from chunks of sulfur that they harvested from a volcano. When sulfur is burned, it creates hydrogen sulfide and sulfur dioxide, which are both extremely poisonous and heavier than air so they would be able to penetrate deep into queerat nests. However, it’s hard to imagine those two things alone have the power to exterminate an entire colony.
Satoru was of the opinion that the Ground Spiders went digging through abandoned cities for plastics that contained chlorine. For example vinyl chloride burns to create hydrogen chloride gas, which is also very poisonous and heavier than air. There are a plethora of gases that are lethal, and even more things that can be combined and burned to create them. Or, most terrifying of all, it could be something new that we have yet to discover.
Purging the gas from the Robber Fly colony took some time.
Even using cantus to displace massive amounts of air was not an easy thing. No matter where you tried to move the air, there would be a force pushing back on it. Creating a powerful whirlwind, Satoru churned up the polluted air closer to the ground and carried it far away, letting the clean air flow back in. The image he created for that must have been quite something.