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 its colony worlds due to violent Warp storms, rendering humanity unable to safely travel the Warp. While the Warp storms died out around M29, it was all but impossible for Navigators to safely guide ships through the Warp. To solve this, the Emperor created the Astronomican, a device which acts as a psychic "lighthouse" allowing travel in a 50,000 lightyear radius of Terra. The Emperor eventually planned to phase out Warp travel completely with the construction of an Imperial Webway, but one guy accidented it.
Following his ascension 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 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. this 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 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 Milky Way like a swarm of mosquitoes to a torch.
Although, considering the Emperor was created by the merging of tens of thousands of powerful psykers, one has to wonder just how overpowered He will be when he returns after having another thousand psykers added to him everyday for the past ten thousand years. In fact, the Astronomican probably doesn't even need sacrifices, just replace the tired psykers with fresh ones, letting the first batch rest, and repeat. Most likely the Emperor knew he couldn't keep up the fight against Chaos forever, so he lied about the need for sacrifices (or some idiot Tech-priest decided its machine spirit was hungry...that's somehow more likely) in order to keep up his strength and fight the good fight forever.
