It was a perfect way to make them stronger in a short time and keep them under control all the time.
The reason why he hadn't used something like this on the most important women of his life, was because while this technique was fast and useful, it was still dependent on him and his cultivation.
Not only that, but the gained cultivation was just like a copy of Tianlong Yun, once Tianlong Yun disappeared, or didn't dual cultivate with them, then their cultivation would stagnate, and fall.
This was the great achievement of the powerful clans, sects, and palaces of the Immortal World, in the intention of growing strong servants to work for them as the Masters.
After all, not everyone in this world could cultivate to a high level in a diligent, and normal way. And if they had a bad generation those powers might even suffer and disappear.
