Before the Night King had amassed numerous Heavenly and Earthly Treasures, each capable of enhancing his Primordial Energy limit, he had never utilized them.
The reason he hadn't used them before was because of the mysterious effects of the Secret Technique he practiced, the Great Dream Heart Sutra.
It allows him to retrain in dreams, and the experiences and skills gained in the dream directly impact his real self.
Thus, the first time the Night King practiced the Great Dream Heart Sutra, it was as if he relived his life in a dream, retraining himself anew.
He intended to wait until he had retrained himself to the most perfect state before using the Heavenly and Earthly Treasures to enhance his Primordial Energy limit.
Otherwise, using the Heavenly and Earthly Treasures and then retraining with the Great Dream Heart Sutra would wash away their effects.
Upon seeing the Reverse Element Skill this time, the Night King had an idea.
