Soul.

As the pain was approaching its absolute climax, the lich inserted a syringe into one of Ark's forelimbs. The syringe contained dark red blood that seemed to be thicker than mercury, as it slowly made its way into Ark's bloodstream. As the blood circulated in his body, the pain that Ark was experiencing almost instantly doubled, but at this point, he was basically out cold already. 

The lich looked on from the side, and he could vaguely see a blood-red hue around Ark's body. Once again, he was absolutely delighted, who would've thought that his experiment would turn out to be such a success. Not only had the embryo of his divine soul started to form, but the embryo of his demonic soul was now starting to form as well. 

"To think that experimenting on a shapeshifter would actually give me such a great effect, he's the first to survive for this long!"