Fun fact: writing up this entry, the fingers of my left hand were accidentally one key to the right, and thus I got "svhsirtsi" instead. That'd be a fun monster name, I think. Perhaps some kind of evil hippo-creature?
Achaierai, on the other hand, are 15' tall intelligent evil birds with four legs. There's not much too them, except the manner of their introduction: a scene from a play written based on the true account of an elven ornithologist named Fiona Silverbane who was captured after being discovered dissecting one of them (she didn't know they were intelligent at the time) and tortured in reprisal. For centuries. (As a fun note: achaierai only live for 30 years ago, which means she was literally handed down for generations.) When Fiona Silverbane escaped, she published what she had learned, and then went insane, thinking that her entire "escape" was just a ploy on the part of the evil birds to break her. "C. Emmet Runn" (yes, really), the "mindsmith" in the insane asylum she was committed to, decided to write a play based on her experiences called "Achaierophobia", which is now considered a classic on the plane of Acheron where said birds most commonly live.
Nothing else in the entry tops that piece of backstory. A bit on migrations, and how they've domesticated rust dragons, but really, the play's the thing.
* The PCs encounter and handily slay a rust dragon on Acheron, just another wandering monster encounter, yeah? Except, of course, it was the "pet" of a nearby flock of Achaierai. The birds are smart and vengeful enough to not go after the PCs directly, but rather, by stalking and observing them for a while, attack the things (and people) the PCs care for, instead.
* An ornithologist wishes to study one of the rare Achaierai migrations between two of the cubes of Acheron. Wary of the birds' evil reputation, as well as the deadly nature of Acheron itself, he hires some protection in the form of the PCs.
* A flock of Achaierai hear about the Achaierophobia play for the first time, and aren't happy. The PCs somehow end up with the gig of protecting Runn from the birds, but the playwright/mindsmith refuses to lay low... in fact, he's got a command performance of that very play to set up, for a fairly prestigious noble!