With a relatively low CR, these horrible bird-fiends are an excellent introduction to the Lower Planes, and a reminder that devils are not the only things to fear in Hell.
Once every five years, two rifts open on opposite sides of the Storr Plains. Flocks of achaierais boil through in a hideous migration, racing from one portal to the other and preying upon the overwhelmed tribes of centaurs that live in between.
In the stews of Meer Ambluth, truly desperate pleasure-seekers are found wandering the streets with the flesh rotting off their bodies. Their drug of circumstance (if not choice) is the confusion-causing black cloud of an achaierai chained beneath the Red Lantern.
Mortal conscripts who fall on Hell’s battlefields are told to beware Hell’s vultures. Despite their bestial appearance, achaierais are as smart as men and can speak. For those malingerers who understand Infernal, it is more than a little disturbing to hear the flavor of one’s own liver being discussed.
—Pathfinder Bestiary 2 7