Royals and Vanity [Supernaturals Underground: Crime Investigators: Book 3] - Supernaturals Underground: Crime Investigators, #3

The demons' plan is worse than we thought. We're going to need help. Unfortunately, our help might kill us. Well, here we are. A War Mage in training, a nervous Seer, and my battle partner, a faerie prince who's also a part-time cat. It seems like we're the only three people in the world who know that if the demons enslave the faerie realm, the rest of the worlds will go with it, spiraling into the terrifying Infernal Dimension. Since going into the fearie realm alone would involve...complications that I can't come back from, we're resorting to an even scarier plan to stop the demons.
And that's getting all the Elder War Mages to help us fight. Which would be easy if they weren't a bunch of ego-heads who could kill us easily. Finding their secret headquarters will be a feat, and so will coming out of this alive.