This was another preparation that Tang Yin had held for Xiao Fei. He was waiting for a moment when she became seriously injured to use the drops of Myriad Spirit Milk.
If he used it earlier when she wasn't too badly injured, then it would have been a waste and even harmful to Xiao Fei.
There was too much energy inside several drops of Myriad Spirit Milk. Her body wouldn't be able to handle all of it.
But now, the excess energy could be directed by Xiao Fei to heal her body. She also had to control the energy to heal her body, otherwise, it would not work.
Before, she would have been too busy with the Everlasting Heaven Flames. But now, she had assumed control over enough of the Everlasting Heaven Flames to focus on healing herself.
A colossal amount of energy entered Xiao Fei, she felt as if she would explode if she did not expel or consume the energy. Either way, she would actually explode if she did not somehow get rid of the energy.