The Dao Heart (道心) is the perseverance in the search of knowledge and truth, the willpower to face and overcome any challenge, the mental aspect of cultivation. With an unshakable Dao Heart even if a cultivator has poor talents or practices a common Merit Law, it is still possible to reach the apex someday. On the contrary, countless genius with high talents but a weak Dao Heart have failed to become Immortal Emperors. In short, Dao Heart is the most important thing for a cultivator, its very foundation.
The inner demon is an entity born from the negative emotions (anger, pride, greed,...) accumulated in the Dao Heart. The inner demon would attack the cultivator during the Fate Calamity or when an oath made with the true fate is violated.
Dao Heart could be transformed into other types of heart, opening the possibility to cultivate in others ways without relying on Blood Energy.
The Buddhist Heart (佛心) is the equivalent of the Dao Heart in the Buddhism. One could cultivate a unshakable Buddhist Heart by severing its mundane ties and being unaffected by emotions, grievances or worries. The Buddhist Heart exists physically and could be removed.
The Devil Heart (魔心) is the equivalent of the Dao Heart for the Dark Cultivators. The Devil Heart is the source of power for the Dark Cultivators, it houses their evil energy and creates their surrounding evil flames. A Devil Heart could be obtained through an incredible painful and difficult process.
The Dao Heart's strength could be used offensively through some arts: