Midnight mirror is one of my favorite Paizo modules, and its thematically a perfect fit for the greater campaign, as well as a way for the party to get an edge of experience and magic items to help them in dealing with the Asmodean Knot in book two. So, I figured I would share the work I did in case anyone else wants to do the same.
Following Raynulf’s suggestion, I dropped the module in between books 1 and 2.
The setting requires some jimmying, but the tallowthroat outbreak fits the city just as well as the small town, and ending it can give the Children of Westcrown some extra early good press.
Prominent landmarks from Karpad (Boroi Manor, Shade Row, the chandlery) can all be dropped into a neighborhood of Westcrown without any real alterations. I changed the Boroi family to House Ghivel, setting up Vuiper for later, but any House you want to focus on will work.
The Shae rebellion I amended to be not centuries ago, but rather decades - the shae and fetchlings trapped in the mirror were rebels against Sivanshin’s power, native shadow denizens that fought to keep the vampire from conquering their dimension. I had already had shae crop up as essentially huntsmen, using shadowgarms as hunting hounds. Depending on how much you want your players to know, the prisoners of the House of Night can know as much or as little about Sivanshin as necessary.
The intriguing possibility here is that your players may make a deal with Nicosar or the fetchlings. When the House of Night crumbles, every living thing is shunted back into the material plane. If the party strikes a bargain with the shadow creatures, they may become valuable allies, even joining the Children of Westcrown and fighting to free their people from the control of the Totemrix. Nicosar, however as a neutral evil being, will only agree to help the Children of Westcrown if he is allowed to kill Stepan.
Default fetchling is a rogue, but I sprinkled some fighters and sorcerers in among Nicosar’s children, as they would have more training than the common fetchlings on the first floor. The three fetchlings on the ground floor are already named, but not the six upstairs. I went with Zasen, Truco, Zorzoru, Yoctivis, Vogne, and Qratol, all generated by Fantasy Name Generator’s fetchling list.