The presence of the horror rank eclipse hiding in Mark's skin threw a monkey wrench on Uriel's plans. His past confidence wasn't baseless, given that he carefully calculated all the possibilities and was ready to face almost anything Mark could have in store for him, but he didn't anticipate the eclipse still being inside him.
Facing Mark who was a heavenly rank hunter like Uriel himself, but also a vampire hybrid was one thing. The strength boost and the instant regeneration of vampires might make things more difficult, but Uriel knew vampires became weaker under UV light and their regenerative ability was cancelled when in contact with wood. However, he knew little about eclipses, let alone a horror rank one and it changed things entirely.
To add insult to injury, Mark let him know that he kept tabs on him this whole time which spoiled the element of surprise.
