The Unbidden
Have you ever looked in a mirror and wondered if the shadow you saw cast behind you did not come from the light in the room, but from the light reflected in the mirror?
The Unbidden are, by what information exists, a race of extra-dimensional entities known as the Eternal. Malevolent, war-like and beyond "normal" comprehension, the Unbidden wage an endless war for something they call the Throne of Eternity, the right to which is measured solely in the energy of all living things, from the most insignificant bacterium to the most powerful intergalactic species. Little beyond this is known by researchers, and those few who have tried to delve into the secrets of the Unbidden have all, without exception, gone insane. One Greater Terran Union (GTU) scientist, who had spent a decade studying stripped “dead worlds” in the Aftermath of the War in Heaven (which the fallen Jaz Gavaz's reckless use of the Jump Drive got the Unbidden's attention of the Milky Way Galaxy), was said to have ultimately gouged his own eyes out and brutally murdered three of his assistants before being shot by Terran Marines who were standing security at his lab. This is thoroughly denied by the Ministry of State Security, along with the other rumor that the corpse of the Scientist continued to speak, even after being repeatedly shot by Gauss guns.
What is known is that when given a specific trigger, usually the overuse of faster-than-light jump drives or hypergate technology, the attention of the Unbidden is drawn. The Unbidden open a “gate” of their own into real space, and launch a genocidal war on all life. While the Unbidden remain the only known faction of the Eternal to ever be encountered, there have been rumors that they are in fact the smallest of the Eternal factions, and that there are least two other, even more powerful versions of this species; All vying for the Throne of Eternity. This claim is generally dismissed as "disinformation".
For the Antares Commonwealth, even less is known beyond what could be salvaged from the Screk Empire's Archives; very little, ultimately amounting to psychic gibberish of destruction across a thousand worlds and possibly even realities/multiverses. Efforts to ask the surviving Screks more about the creatures has been met with a mixture of almost religious symbolism against evil, or in some cases a near generational traumatic response, to such a point it is considered impolite to even mention the Unbidden in the presence of a Skrek.