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 (which may have been caused by the all the murder-sex being caused by the Eldar), 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 radius of Terra. The Emperor eventually planned to phase out Warp travel completely with the construction of an Imperial Webway, but certain things came up.

Following his ascension to the Golden Throne, the running of the Astronomican fell to the Adeptus Astronomica. Given the sheer amount of psychic power contained within the device, the Emperor's mind is used to direct the power the Astronomican. To keep it running, one thousand psykers are rounded up and sacrificed to power the Astronomican. 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: it's attracting the Tyranids to the Milky Way like a moth to flame.