Phantom told Ye Xiao how Yu Rong feed it a pill when she captured it. That pill was something else, it was not only very attractive to Phantom, but it also had the attribute to raise Phantom's limit of future strength.
Normally, beasts like Phantom could only grow to Chaos Spirit Saint Realm at their peak, however, that pill broke this limit and now, Phantom could grow much stronger.
As for why Yu Rong fed Phantom that pill, neither Phantom nor Ye Xiao had any idea.
Ye Xiao suddenly suspected that Yu Rong might be keeping an eye on him, but then he shook his head. This might not be true, after all, he has already lost his value in Yu Rong and others' eyes.
If someone might be keeping an eye on him then it would be Yu Tian. Then again, that might not be true. After all, Yu Tian never contacted Ye Xiao again to ask him why he is not cultivating the Primordial Chaos Grand Mysteries.