Earlier, when he heard outside that the Underworld possessed thunder techniques, he had guessed that the secret manual for them should be inside the treasury. Upon entering, he had immediately used his spiritual sense to search the place, and just as he expected, he found this volume of the "Jade Firmament Thunder Technique".
He opened the pages and began to browse roughly. After looking at a few pages, a mix of shock and joy suddenly surged in his heart.
As it turned out, the "Jade Firmament Thunder Technique" was divided into two parts. The upper half used oneself as a cauldron to absorb thunder qi and strengthen the body, while the lower half was the thunder array outside.
Chapter 729