Aphotic is a Grand Dao conceived by Immortal Emperor Yao Shi from the mantras of the Death Scripture, one the Nine Grand Heavenly Scriptures. He didn't use it himself and left it to the descendants of the Myriad Bones Throne.
Only in the Current Era, Di Zuo, the Throne's current Prime Descendant, was able to explore the mystery of the Dao and use it. But even he hadn't completed it, and required to use a tremendous amount of Blood Energy and Longevity Blood to activate it.
Di Zuo used the Aphotic in the battle against Li Qiye and managed to trap him inside. Li Qiye tried to use different techinques to free himself, but failed and was forced to show and use his 12 Fate Palaces to obliterate it.
Aphotic creates a field of darkness around the user and his/her enemies; a domain separated from the rest of the world with no dao or universal laws inside. Inside of the Aphotic Domain all dao and Merit Laws, including Emperor's Laws, lose their effect and cannot be activated.