Adam had come across this sphere of light several years ago. But there were two reasons that he still had not consumed it.
Firstly, the size of the soul was too big for him to devour. At the time, he figured that he had to become at least a Rank 3 - Mana Vortex Magus to absorb the soul.
Secondly, and most importantly, he believed that even if he did become a Mana Vortex Magus, he would never abosrb this soul becuase of the dark and repulsive aura that it radiated.
At the time, Adam feared that absorbing the soul of such a wicked and ominous being might have some effect on him. No matter how minute the changes would be, he didn't wish to absorb it.
Now, however, he could come up with two reasons why he must absorb this soul.
Firstly, the aura it radiated had some similarities to the aura generated by the blood ritual magic in the Duskfall Masquerade as well as the devil tongue inscribed on the walls of the black-and-white tunnel.