Sol's hair had and mana had returned to normal after he used [Forced Sun] and [Pulsar] on Dr'ul; not as a measure of his strength waning but rather his intent was to taunt her, he wanted to see more of this power, she was already fighting on the level of Orion which in itself was beyond amazing, considering Orion was the most powerful demonic being from the dark universe, topped only by Darkness and Void.
If this was him from two days ago fighting her he would have easily lost to her, but Godslayer had broken all of his limiters ramping his power several times that of what it used to be; and that is what fascinated him.
The fact that she was able to channel all the negative emotions and trauma of Arkadia into herself, all the hatred and anguish she had converted into fuel and supercharged herself thereby evolving into a being he couldn't properly understand despite having the most broken power at his disposal.