I can't seem to find any regarding canon material on either group of Xenos. Implications are that most (heretical) information has been stricken from Imperial record!

I like the idea of one or both of them being mysterious and vague; it makes them seem far more sinister. I may add some surviving Egarian city or planet in my game. I was wondering, however, if I need to create the species from scratch. Most material relates that the Yu'vath (of the Calixis Sector) corrupted the Egarians which somehow lead to their disappearance / extinction. Also, all that remains of the Egarian Empire are their maze cities and various bits of creepy, cool technology.

I'm extrapolating a hybrid of the Grays (of X Files and UFO folklore) with worhshippers of Tzeentch. Seems like a natual fit; shifting urban mazes, flying disks, flashing multi-colored lights, telepathy, fiendish abductions, lost time, anal probing, cattle mutilations, and intricate, inscrutable motives and plots.

Apparently, the Yu'Vath might look a bit arachnoid (http://z6.invisionfree.com/bljunkies/ar/t1871.htm), though I don't know. Egarians pulled a nothing on the image search, so no clue. Neither race appears on the 40k wiki, either, so…

I think someone mentioned at some point somewhere that the Yu'vath looks kind of similar to Lovecraft's Mi-go. But it can also be total BS of course.

There was a Call of Cthulhu modern / FBI source book thing, from the 1990's, called Delta Green. In it the Grays (again- of X Files and UFO lore) where actually created by the Mi-go in order to "communicate" more easily with humans, since the brains-in-a-can thing weren't yielding favorable results. Some humans go nuts easy, I guess. I think I'll stick with my original idea of extra-creepy Grays and the Mi-go (as the Yu'vath), for the sake of nostalgia

In one of the books… Bestiary maybe… it specifically says that they'd reached a point where no two looked or acted alike because they were so corrupted and mutated. That is, anyway, how I always consider it in my mind.

My Egarians are insectoid, kind of like a preying mantis crossed with a dragonfly. They were destroyed by the Yu'Vath after being drawn into a battle against them. My players were guided by them using a combination of the Star Mirror from the core rulebooks scenario and the nexus points of the heathen trail from "Lure of the Expanse". Who needs the Eldar in everything after all :D The Egarians communicated psychically and guided the players to stop the Yu'Vath returning to the universe and destroying everything.

Naturally there was a hilarious and incredibly ironic twist…….

The important thing with these games is you don't have to be held exactly to whats in the books, the Yu'vath and Egarians are deliberatly vague so you can have fun with them and make them what you want and need for your games.

I haven't been able pick up Koronus Bestiary yet, but I thumbed through a friend's copy a while ago, and I could swear it had at least one picture of a Yu'Vath (it looked kind of like a Cloaker from D&D, but with spikes along the edge)…?