The Thunder Emperor felt like he had gone mad.
Initially, when the Wise King found him and entrusted him with such a task—as a citizen of the Human Central Empire, he really had no reason to refuse. But then again, the task of subjugating the Ice and Snow Kingdom wasn't necessarily meant for the Thunder Emperor alone; there were other strong individuals who could take action. If he were unwilling to step in, the Wise King likely wouldn't blame him.
However, the Wise King's constant praise, saying "You are indeed the most powerful mage of our Human Central Empire, master of thunder and lightning, who can stop you" and such flattery made the Thunder Emperor, already feeling invincible, slightly floaty. Indeed, if talking solely about magical prowess, he might not be as good as Adriana, but when it comes to lightning, in his lifetime, the Thunder Emperor had never been surpassed.
Even Adriana, if confronted with his thunder magic, would definitely be no match.
