Memory
Sam is in the middle of the soul extraction process, but all of a sudden he felt like something hit him.
It was not a physical hit. It is a hit to his soul. He could feel it. At this, he is using his soul to extract her soul out while restraining her mental strength completely.
But suddenly, the restraints of the mental strength seemed to be loosed up for no apparent reason or to be more precise for the reasons he couldn't understand or imagine.
The attack is light, but he halted in his tracks to make sure that nothing dangerous could happen to him in this process. Anything that is involving one's soul is too much danger. In fact, it is dangerous than playing with a spiritual core.
Because there are a lot more ways to heal or even regrow a spiritual core compared to the healing methods for the soul.
Last time, his soul barely got into the shadow realm for a few seconds and he was out cold for two months straight.