Dantai Yu could not understand how lightning spells could possibly be used in such a deep underground place where the Yin energy was so dense.
Generally speaking, lightning spells required an abundance of thunder and lightning factors in the environment. Thunder was a natural force; the caster merely used their spells to trigger it and guide its path.
Unless the caster was directly releasing their own energy, which was rarely done because it would exhaust their magical power at an alarming rate.
Chapter 208