Baron Kernaghan is the time-locked main villain of the series and tied for 3rd with Luke Callindor and Fizzle. He is the great darkness that was wiped from the memory and history of Windemere. His return will revive his legacy, which can cause insanity in mere mortals. Also, the gods have their Law of Influence this time, so they can’t step in to stop him. This is where the champions come in. Arthuru is also the father of Stephen the sadistic madman that is after Nyx.
On the other side of the conflict is Gabriel the God of Destiny. He has created the champions and several of the Baron’s henchmen because the game has to be even, but he’s definitely choosing sides. There’s a history between these two has been hinted at and a few clues dropped throughout the books. Sleeper of the Wildwood Fugue definitely has a lot about them and the early conflict that placed both of these once mortal men into their current roles. So who better to put in a room together? Especially since they rarely get any book time.