The anatta mental state is a stage of cultivation where the mind is in a tranquil and calm state, completely free of the constraints caused by one's surroundings.[1] Anatta is the Buddhist concept of “non-self” (more information).[2]
During the Lord of Nether's Disciple Selection the test on the eighth level was to reach this state of cultivation while within the Black Infernal Tower.[2] Within the tower were powerful black flames that could cripple one's cultivation and extremely powerful Demon Beasts chained up, making it more difficult to concentrate.[1]
During Venerable Redsoul class, he taught to create Spiritual Flames. In order to create a Spiritual Flame, the Soul Realm must reach the Anatta Mental State, then gather their intent onto their palm.[3]