The Xenarch are a xenos species populating certain areas of the northen side of the Galaxy. They are described by Magos Biologis Sharle Darvus as an isolationist, warp-worshipping species, which makes it a clear target for extermination for the Imperium of Man. However, the difficulties of travelling to the areas where they supposedly have their little empire and their hermit-like attitude makes it very difficult to actually find their planets. We know that the Black Templars, led by Helbrecht organized an extermination campaign against the Xenarch Empire in the Sigilare Nebula in 989.M41, though considering all of the information we have, it's very possible they are still around somewhere.
Their biology is largely unknown, other that they can produce electricity within their bodies. Their weaponry is used in conjunction with this ability, creating lightning weapons such as the Xenarch Conduction Spines and the Xenarch Death-Arc. These devices are of particular note thanks to their energy-storage capabilities, putting similar human devices to shame. They are so good, in fact, we could assume the Adeptus Mechanicus would love to get their hands on a couple of these things, in particular the electro priest sects, considering how enamoured they are with lightning and energy.