Astronomican
The Astronomican is a device created by the God-Emperor of Mankind in preparation for the founding of the Imperium of Man.
Following the Golden Age of Technology, humanity was cut off from it's colony worlds due to violent Warp storms, rendering it unable to safely traverse the Warp. While the Warp storms died out around M29, it was all but impossible for Navigators to safely guide space ships through it. To solve this, the Emperor created the Astronomican, a device which acts as a psychic "lighthouse", allowing warp-travel in a 50,000 light year radius of Terra. The Emperor eventually planned to phase out Warp travel completely with the construction of an Imperial Webway, but one guy accidentally ruined it, which led to an immeasurable amount of rage due to Not as planned, both from Big E. himself and the fan-base alike.
Following his entombment to the Golden Throne, the running of the Astronomican fell to the Adeptus Astronomica. It's overseen by the Master of the Astronomican, who has a permanent seat on the council with the High Lords of Terra. Given the sheer amount of psychic power contained within the device, the Emperor's mind is used to direct the power of the Astronomican. To keep it running, one thousand psykers are rounded up every day and sacrificed to power the Astronomican. Unlike the ones sacrificed to maintain the Golden Throne, these psykers are trained to keep it running and consider it a holy duty to sacrifice their lives *BLAM*, imperial fanatic propaganda. The device is a trash can for psykers that are worn out, insane, or just happened to piss someone off. By keeping it active, humanity has a guaranteed way of navigating the Warp and keeping the Imperium "functioning".
There's actually way more than one thousand psykers inside the Actronomican Chamber of the Hollow Mountain. The mechanisms of that chamber gradually drain each victim of all memories, thoughts, emotions and eventually the soul itself through a course of a few weeks, or in case of the most unfortunate ones - months, and all this time for them is filled with extreme, incomprehensible, mind-blowing, "I wish I was abducted by the Dark Eldar" pain, until all what left of their souls cease to exist (so no afterlife for them) and all what left of their bodies crumbles to dust.
Where the light of Astronomican collides with a potent warp-storm, like the Eye of Terror or the Maelstrom, the mix of both creates a rim of space fucked up so hard even daemons are afraid of it. These burning walls of warp-fire are inhabited by warp-spirits that could be described as flaming angels, just like Chaos daemons, don't mind slaughtering, raping and eating chaos worshipers. These "angels" are known to cleanse their territory of all life oldcron-style, no matter it's allegiance, but unlike daemons, these spirits cannot be reasoned with, as they are as insane from millenniums of intolerable pain and suffering probably just as the source that gave them life.
There's only one real drawback to having the Astronomican (you know, aside from whole sacrifice of a thousand innocents per day): it's attracting the Tyranids to the Terra (they were already on their way to the Milky Way due to the Pharos) like a swarm of mosquitoes to an infra-red torch so it should really be renamed to the Astro-nom-ican. Is this a draw back? Who knows? Maybe Big E. needs the 'nidz to set him free from his Golden prison.
Although, considering the Emperor was created by the merging of a bunch of powerful psykers, one has to wonder just how overpowered he will become when/if he returns, thanks to all those sacrifices everyday for the past ten thousand years. The fact that the psykers are sacrificed and are not allowed to rest between sessions is also a typical source of grimdark.
