The Yu'Vath were a xenos species hailing from the region that is now known as the Calixis Sector (formerly the Calyx Expanse), in Segmentum Obscurus. Until the 39th millennium they possessed a formidable empire, but they were ultimately destroyed by the Imperial Crusade that preceded the formation of the Calixis Sector. It is generally assumed that the Yu'Vath were rendered extinct as a species, but scattered remnants of their empire can still be found.
Though many records of them were expunged by the Inquisition, it has been confirmed that the Yu'Vath were of "varying appearance," with soldiers from one battle displaying entirely different morphologies from those of another, despite being the same species. They also had an affinity for crystals, including an alliance with a species of sapient, hive-minded crystals. It is believed that while the Yu'Vath themselves were few in number, they each held court to a vast number of slaves who existed only to further the perverse, cruel whims of their masters.
There is no official imagery of a Yu'Vath, but some of the Fantasy Flight Games Warhammer 40k RPG material, for which this race was created, portrays their battle constructs as crystalline spheres coruscating with dark energies. It's a lot worse than it sounds.
Prior to the arrival of the Imperium, the Yu'Vath had created an empire that dominated the Calyx Expanse, and possibly parts of the Koronus Expanse as well. They brutally destroyed and/or enslaved almost every other species they came across. It is confirmed that they were warp-worshipers, and many of their weapons and strategies depended on sorcery to function, to the point that a baseline human might go insane just looking at their warp-infested ships.
The Imperium's arrival meant bad news for the Yu'Vath though, since warp + alien = DOUBLE HERESY!. During the Angevin Crusade, it was reported that an unusually high number of exterminatus attacks were carried out, mostly due to the inevitable taint of Chaos after millennia of warp-worship. Further exploration and the continuation of the campaign revealed that, while the Yu'Vath had a vast number of slaves, there were also not quite as many inhabited planets as initially believed, which made the Ordo Xenos think that this species had been on the decline for millennia. Yu'Vath ruins are still occasionally discovered on uninhabited planets, which further supports this theory.
By the end of the Avengin Crusade the worlds of the Yu'Vath and their xenos subjects were all destroyed, along with most of their technology.
Two things make Yu'Vath technology especially hideous and blasphemous compared to other xenotech. First, they are heavily based on the Warp to work. Almost any piece of their tech needs to connect to the Warp to function, either as a power source or as ammo. Second, they used a ridiculous amount of Aberrant Intelligences, or "synthetically constructed spirits" as they called them. Many in the Ordo Malleus think that these weren't just artificially created intelligences, but were instead bound daemons, comprising many of the Yu'Vath's constructs. Officially the Imperium is very interested in destroying every piece of Yu'Vath xenotech that can be found. However, as dangerous and proscribed as it is, intact Yu'Vath tech is still a tempting prize for the rich, powerful, and/or insane. That being said, it must again be noted that collecting Yu'Vath technology for study or any other reason is definitely not recommended. Even the least of their artifacts are unpredictably dangerous and poorly understood, they are often aggressively hostile to their users, and there are often steep consequences for using them. For example, there have been reported cases of Yu'Vath artifacts, that once activated, pierced holes on the spaceship's Geller Field to connect to the Warp- with obvious results. In comparison to the possibility of eternal damnation, the guarantee of a visit from the Inquisition would seem rather pleasant.