Ace suddenly felt a foreign Qi enter his brain, and he felt his head split as a huge pain attack him. He felt this Qi was drilling inside his knowledge sea forcefully was Gordon's martial sense!
He never thought this soul search would be such a vicious method, it wasn't like his gentle soul probing at all.
Gordon wanted to use only superficial soul search on Ace since the full prowess of it will turn him into a 'vegetable'. Furthermore, this technique was for a soul realm cultivator, but he was using it on the river core realm, only he can do this since his real cultivation was much higher than his clone, but this action has its consequences. Like it would take a huge toll on Gordon's body and mind, or even cause a back backlash if he used it for an excessive amount of time.