The mechanics of Terik's planning board and the cost seems completely out of left field, considering she apparently has infinite lightning storm powers and I assume teleportation for objects. Combined with Vincent not choosing to lose his magic, and Mason being the one to kill Stephan because of Terik, if Vincent hadn't trained at all and got killed immediately it would have made no difference in the outcome of the battle, meaning the 3 books of training was effectively pointless in the end. He doesn't even keep his magic, so it's not even theoretically useful in being an EC agent or anything like that. Stephan didn't need to be weakened by Vincent, or really at all since Mason snapped his neck even through his shadows with a full bracelet.
The Divine aspect was also anticlimactic. This rare and unknown aspect just lets you get possessed by gods when they decide to possess you? How is that an elusive aspect of magic?
I've been worried how the ending would land with the meandering of the third book and all the training and evolution of the first 2, but this just does not land for me. I've followed all 3 books as they were written, and I'm sure plenty of people will still enjoy this ending, but I hope my reasons why I think it was weak are clear. I always felt the main strengths of this series was the mystery in the world building and the plot, and while the world building was still pretty good I found the plot really fell through in this last book. I still think that a second revision going back through and probably shortening it to 2 books would make this a stellar series though. I wouldn't spend time on constructive criticism if I didn't think that the core of the work was good and that there's potential here. You've made a really good serialized story here that could be edited and revised into a book series I'd happily buy.