WHat exactly do we know about the Yu'vath? i went digging for information but didnt find much
The Koronus Bestiary for Rogue Trader has a 13 page section on the Yu'Vath, including write ups on four different constructs. There are hints and snippets scattered throughout the Dark Heresy background as well, but I'm pretty sure the KB is the single biggest block o' published text on the Yu'Vath.
Here's the short version: Yu'Vath are Warp-worshipping xenos. Their empire was very ancient, hundreds of thousands of years old at least. Yu'Vath are confirmed extinct in the Calixis sector, and believed and hoped to be extinct everywhere… yeah, good luck with that. Yu'Vath sites are also found scattered across the Koronus expanse. Their technology draws strongly on the warp, and often has a limited ability to self-repair. Yu'Vath items look like amalgams of crystals and fossilised bone, often crackling with dark lightning. The items frequently have the ability to manipulate the minds of sentients in their vicinity, and there are numerous accounts of some unwitting fool picking up a Yu'Vath item and subsequently being dominated and forced to fulfill an aeons old agenda. Also, the Yu'Vath had many slave races, who were "modified" to be more "suitable" for whatever blasphemous task the Yu'Vath wanted them to perform. Some of these slave races do survive to the present day, including two in the Calixis sector itself.
Oh, and those four constructs in the KB? They're pretty damn hideous by Rogue Trader standards. You'd best have a couple CSM's along if you plan on fighting one.