You Dug Out My Daughter's Supreme Bone? I'll Abandon the Imperial Pass and Become a Demon Lord

[Darkening System + Invincible + Cathartic Action + No Forgiveness + Demon Lord] For a thousand years, Qin Zimo guarded the Imperial Pass. All he wanted was for his wife and daughter to live in peace, and for humanity to enjoy peace for generations to come. For that, even though the System kept telling him that turning evil and becoming a demon would let him attain Emperorhood, he chose to ignore it. Until a blood-written letter from his daughter, Qin Yuan, landed before him. “Daddy, where are you? Mother has switched to the Path of Ruthlessness. She doesn't want Yuan Yuan anymore.” “They're so mean, and they want to dig out Yuan Yuan's bones. Daddy, Yuan Yuan... misses you so much!” Qin Zimo froze. He had risked his life defending the Imperial Pass, and in return they had turned their butcher's knives on his daughter? A thousand years of protection, and this was the thanks he got! In that case, whoever wanted to guard the Imperial Pass could do it! He fought his way back to the Vast Heavens to demand justice, only to be met with curses. That day, he finally fell to darkness and descended into the Demon Abyss with his daughter... Thirty years passed in the blink of an eye. His daughter grew into an Empress, and he became the Lord of the Demon Abyss, a name that struck terror into the hearts of all. It was also then that the myriad races dropped all pretenses and invaded, bringing the heavens to their knees. Only then did those old acquaintances panic and begin searching everywhere for him, weeping as they begged him to send out his troops and quell another catastrophe...