Mag'ladroth, the Void Dragon is one of the most powerful of the C'tan. It's current whereabouts are the subject of rumor and speculation.
Official Canon Edit
Not much is known for certain about the Void Dragon, but multiple theories about it exist. The Void Dragon is mainly associated with technology, leading to the speculation that it is in fact the Dragon of Mars, a being that is secretly kept imprisoned by the Adeptus Mechanicus. According to legend, the Emperor himself, under the guise of Saint George, defeated the Dragon on Earth and later had it imprisoned on Mars to give the Mechanicus its power over technology. But if the Dragon awakens, it could mean disaster for humanity.