Some of this stuff is spoiler-ish, but most of it is borderline. Read at your own risk.
By the way, if you like super weird, uncomfortable satire/humor, there's this series of videos by the comedy group Wham City. It's called the Children of the Mirror. I promise the concept of Hounds of Tindalos is crucial to understanding what the hell is going on in this series (which follows the True Art is Incomprehensible trope pretty well while also maintaining surface level entertainment):
https://www.youtube.com/watch?v=uFw0b8t4ICM&list=PLmu6JBK17BzjH0HBjhdUXc2IfrCRQGhcD Interesting bit is that the Hounds weren't in Lovecraft's original works, but were added in by a different author, Frank Long, one of Lovecraft's good friends. I think it was the first time HP let someone else write for his setting. Lovecraft canonized it (what is and what is not canon in Lovecraftian fiction is always debatable, because so many other authors contributed to the universe over the past hundred-ish years. Pretty much EVERYTHING is canon as far as I'm concerned, Derleth's attempt at codification be damned) at some point a few years later in one of his longer pieces. I forget which.
Anyway, the Hounds are entities that are pretty much the perfect Pursuit Predator. If you catch the attention of one of them, they will hunt you FOREVER. There is basically zero escape. They can travel through any sharp-ish angle (hence, I assume, Eb's use of the term "Cornerhound"), like the end of a hallway.
You wanna know the best part? The really awesome, tinfoil-hattish part? The way you catch their attention is by
traveling through f*&^ing time. The presence of one (or more) during Peace Talks is going to launch thousands of Time Travel WAGs (unless it's explained thoroughly in the book. Just saying "Ah, a mortal must've summoned one" won't be enough).
I mean... why specifically one of those, even if it's an Outsider? It's one of just three named Outsiders in 15 books—Walkers, Nemesis, and Cornerhounds (the "mistfiend" from Turn Coat I took to be a Nevernever creature that had been infused with Mordite rather than an Outsider itself. If it is indeed a true Outsider, then it's four). You very rarely hear Eb (or anyone, really) talk about different species or categories of Outsiders, just kinda "oh, that was an Outsider." Harry describes the scene at the Outer Gates as one in which most of those fighting have too little in common to really categorize. So I posit that if Eb can recognize this particular type of Outsider on sight, it must have appeared on the mortal plane often enough to warrant naming beyond "Outsider, Type 87645."