Chapter 206 - Uto query

I’m a bit confused about a part of the last chapter(s). Seris said Arthur stabbed auto’s core so he couldn’t use mana. I get this part, but then how has he been attacking with shadows since it was stabbed? Also, before this chapter it looked like Uto was winning when he smashed Arthur and Sylvie into the ground, so how was he suddenly winning? Unless I am mis-interpreting something