The Comet is a name given to an artificial comet within the Sol System.
Built from unknown origins during the Dark Age of Technology, the structure orbits the Sol System and appears over Terra every thousand years. During one such appearance over Terra, the Unification Wars were coming to an end and the Emperor decided to intercept The Comet and modify it as a shrine for his victory. Subsequently during the Great Crusade, the Word Bearers built a shrine to worship the Emperor on the structure. After the Word Bearers were reprimanded at Monarchia, Sor Talgron was secretly charged by Lorgar to dismantle the shrine, though in truth this was just a cover story to conceal his sabotage of the Imperial Palace. During the ensuing Horus Heresy, The Comet was cleared by Sigismund and a force of Imperial Fists which massacred the remaining Word Bearers on the structure.
During the Solar War shortly before the Siege of Terra, The Comet proved decisive for the Traitor war effort. A force of Thousand Sons under Ahzek Ahriman as well as Word Bearers and their cultist-slaves landed on the structure during the height of fighting across the Sol System, clearing it of the small Battle-Servitor garrison Rogal Dorn had left behind. The Thousand Sons and Word Bearers sacrificed the cultists and completed a ritual begun by the Word Bearers years previous, opening up a Warp Rift over Luna from which spilled thousands of ships led by Horus himself.