Since they had already been gathered into an army, they would certainly be discovered if they were to move together. Most of the soldiers hid in the forsaken towns and mines not far away from the "Great Iron Plants" and would turn into a surging torrent below the ground at an order.
Only Long Yangjun, Li Yao, Li Jialing, and the few experts in the army were bold enough to approach the Great Iron Plants and contact the local Nepenthe believers who were directly commanded by the Grand Guardian.
The underground worlds were essentially a giant maze made of countless caves that were connected through tunnels and gaps. Theoretically speaking, it was easier to defend than to attack. As long as an elite troop was sent to block the critical tunnel between two caves, nobody would be able to move between the two places.
