Surprise, surprise!!!
Yin-Yang Harmony Sword Technique!
It was one of the unique sword techniques because it allows him to understand the power of Yin and Yang. The same power that he has been dealing with for a long time.
Every time he dual cultivates, he could feel the power of Yin and Yang but he never truly understood the meaning behind these two forces.
Yin and Yang represent the extreme opposite of anything. But, at the same time, one can't exist without another. Just like a human. You need a man and a woman to continue humanity.
One can't live alone because when one dies, there will be no one in the future and one can't be born in the first place if there is only one of them.
This swordsmanship deals with the harmonious relationship between Yin and Yang. If two are combined together, then the power will be absolutely over the top but even if they aren't combined, they will still be stronger than anything else.