The best Dao Heart refining technique goes to the Mental Matrix technique of the Myriad Phenomenon Sect. It is difficult to start it, but there is no chance of ever losing control or experiencing a backlash with it.
As for the Sky Raising Technique of the Four Directions Sect, it can't be the best because the sky in the mind can become too heavy for the giant and crush the giant. This is a backlash that will lead to the breaking of one's Dao Heart and the immediate loss of control.
If their Dao Heart Breaks, their willpower will fall, and they will lose control of their Core Metac. Then their Core Metac will take over their body and make them kill every human that they encounter.
This is what's called a demon. It is a cultivator who has lost control and whose mind is filled with only the slaughter of every human being.
