Yu'Vath
The Yu'Vath is a xenos species living on the Calyx Expanse, in Segmentum Obscurus. They developed a small galactic empire by the 39th millennium, but the arrival of the Imperium of Man to the area basically meant its extinction. Its fate is currently unknown due to the general lack of information and difficulties to explore the region.
We have no idea how did they look, since almost all of the records of them were expugned by the Inquisition.
History
Previous to the arrival of the Imperium to this area of the galaxy, the Yu'Vath created an empire across the stars, destroying and/or eslaving almost every other species they came across. It is confirmed they were warp-worshippers, and many of their weapons and strategies depended on sorcery to function, to the point of people losing their minds by just looking at their warp-infested ships.
The Imperium's arrival meant bad news for the Yu'Vath, since warp + alien = DOUBLE HERESY!. During the Angevin Crusade, it were reported an unusually high number of exterminatus carried on, mostly due to the taint of Chaos spread throughout most of them after millenia of warp-worship. Further exploration and the continuation of the campaign revealed that, while the Yu'Vath had a remarkable amount of slaves, there were also not quite many inhabited planets as initially believed, which made the Ordo Xenos think this species had been on the decline for millenia. Many Yu'Vath ruins pop up on uninhabited planets, which further supports this theory.
After the Avengin Crusade was over, all located Yu'Vath and xeno allies' planets had been purged, and most of its technology destroyed. However, it's close ties to the Warp meant their tech remains quite a prized possession to have, if one doesn't fear being corrupted by the immaterium.
Technology
Two things make Yu'Vath technology especially hideous and blasphemous from 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. They even had the equivalent of Daemon Worlds, where they could bend the rules of physics as they wished, but not enough to avoid destruction by exterminatus. Second, they used a ridiculous amount of Aberrant Intelligences, or "synthetically constructed spirits" as they called it. Many on the Ordo Malleus think those weren't just artificially created consciences, but instead captured daemons, making many of the Yu'Vath's vehicles Daemon Engines. These two things make sure that the Imperium is very interested in destroying every piece of this xenotech they can find, as fast as possible. Also, collecting them for study is not recommended, as there have been reported cases of those things making holes on the spaceship's Geller Field to connect to the Warp, with obvious results.