WAG: Nicodemus is lurking

[Bad Alias]

That means he should be back after the wrestling book, which will be a grail quest (Harry is a completist)

That would be a nice ending to their relationship. First, he comes for Harry. Then he tries to manipulate Harry. Then he gets Harry on his team and works with him until he betrays him. Finally, Harry's had enough, finds out where Nicodemus is, and goes gunning for him.

[Conspiracy Theorist]

It would also be a resolution to Nicky and the Fallen’s entire story line. Nicky has always to himself, been the hero of his own story we get to understand that story.

[Snark Knight]

1.  Nicky and crew foresaw Ethnius eventual arrival and wanted to stop it...OR
2.  Nicky is in league with the titan but failed so badly that the her only choice was to appear early before the Accord Members could officially band together under a new treaty.

I'm not sure either are the case. I think despite all his bastardry, he's also militantly anti-Outsider. I don't see him throwing in with anyone who's in league with them. I don't even think he would have been willing to do business with the Fomor to buy attack dogs from them in SmF if he'd known they were teaming up with the Outsiders for a push like this.