After entering the ninth floor, The Conquerors found themselves surrounded by sea water. They at first tried to swim upwards to get out of the water but soon discovered that after a certain height they were unable to go up anymore.
So for this floor they needed to fight while getting submerged in water. But before thinking about combat they needed proper equipment to fight under water as none of them were suitable to live inside the ocean.
Rein quickly took out various pieces of equipment they would need to survive under water. With these not only they would be able to breathe under water but they would also be able to walk normally and did not need to swim anymore.
Even then with the surrounding water obstructing them, the overall combat capability of the whole team was definitely less than normal. So they needed to be extra careful on this floor.
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