Originally, all of this was Zhou Xun's plan.
From the moment he met Sun Yuanqing, Zhou Xun devised his scheme for action.
Sun Yuanqing was a Late-Stage Foundation Establishment Cultivator with mana several times his own.
Moreover, as the direct disciple of Master White Cloud, he had plentiful means and magical treasures; engaging directly in Magical Combat, Zhou Xun could maintain not losing.
But over time, his mana would be depleted, and he would end up captured.
Thus, Zhou Xun decided to use Master Celestial Thunder for a sneak attack, which was comparable to a strike from a Foundation Establishment Peak Cultivator; if caught off guard, Sun Yuanqing might perish.
Those Yin Fire Thunder were meant to cover up the replica of Master Celestial Thunder.
However, Sun Yuanqing's special identity likely means he possesses life-saving cards; if that replica did not succeed, Zhou would use his physical strength to sneak attack closely.
