Balance

The 'black wisp' was the name Gaia and the priestess used to refer to Khaos. Hearing things related to his cultivation did interest him very much. 

"Please, priestess, tell me what you know."

Apparently, now that the high priestess has transcended, she knew more about Gaia's plan for him. 

At first, Gaia thought the black wisp influence would be advantageous to his growth but gradually she realized the two will not be able to coexist together. 

Especially when his fey bloodline starts to come into the equation. 

The fey as the protector of Gaia has affinity in the natural elements. It only enhances his three other elements but will conflict with the darkness element which is his core right now

Hence as the balance has been broken the previous arrangement with the darkness as his main element will no longer be feasible for him.