The Void Dragon, sometimes referred to as The Dragon, though its proper name is Mag'ladroth, is one of the few remaining C'tan, one of the Necron's Star Gods, in the Milky Way Galaxy. The Void Dragon, or a large C'tan Shard of it, is believed by some Tech-priests to be the actual Machine God venerated by the Cult of the Machine of the Adeptus Mechanicus. A master of the material realm, this particular C'tan was a figure of oblivion and destruction and its created warriors were nigh invincible.
No C'tan Shard of this C'tan has yet been encountered, and it is possible that the Void Dragon has remained whole through the eons and its essence was never captured within the Necrons' Tesseract Labyrinths. This is a truly terrifying prospect for the galaxy if this entity were to awaken. There may be a connection between this entity and the Dragon of Mars, an unknown entity which was imprisoned beneath the surface of Mars by the Emperor of Mankind before the start of the Age of Terra.
The Void Dragon, if it actually exists, is arguably the most powerful surviving C'tan, particularly after the banishment of the Nightbringer's Phase Scythe to the Warp by the Old Ones and their Eldar servants during the War in Heaven millions of standard years ago, although the Nightbringer seems to have had a much more sinister effect on the galaxy as a whole.
Mag'ladroth is named "The Void Dragon" in Eldar Mythology, which portrays it as a figure of oblivion, wanton destruction and devastation. It was a master of the material realm and its warriors were practically invincible during the War in Heaven. While eating stars it takes the form of a cloud of dark light. In an Imperial transcript of a Vox-communication between an Inquisitor named Horst and a Tech-priest named Alagos, the Void Dragon is described as the most powerful of all the ancient Star Gods, having once possessed the ability to create almost invincible warriors.
According to the ancient Eldar legends, these warriors could channel lightning into their foes, though these lightning attacks were ineffective against the "Iron Knights" that the Eldar hastily created to battle against the ancient Necrons. Having gained a respite through the forging of the Iron Knights, the Eldar God of the Forge, Vaul, created the "Talismans of Vaul" (Blackstone Fortresses). These too were powered by the souls of Eldar Seers and were used by Vaul to attack the Void Dragon directly.
Here the Eldar legend ends, and there is little evidence as to the outcome of the legendary battle, other than the stated opinion of Inquisitor Horst, which was that the Eldar attack had failed. At the end of the Eldar's mythological version of the War in Heaven, the Dragon created the Warp Suppression Pylons and the Necrons spread a network of them around the galaxy in key locations. The purpose of these devices was to block the Old Ones' source of psychic power, creating an unbreakable barrier between the Warp and realspace that would forever cut off the Old Ones and all of their servant races like the Eldar from their psychic abilities. However, this project was interrupted and the mechanisms were never finished.
The Void Dragon at present is widely whispered to be slumbering in some form beneath the surface of Mars. If what the Adeptus Mechanicus first worshipped as the Omnissiah is truly this sinister xenos entity, as some Tech-priests seem to believe, there is no telling what the ramifications of this discovery could be. While only speculation at best, it is believed by experts knowledgeable on this topic that the Void Dragon is on Mars, likely in the Noctis Labyrinth. This is believed because of the suicide rush by five Necron Shroud-class light cruisers that succeeded in penetrating the planetary defences of Mars and then landing on the Red Planet near the Noctis Labrynthus in the late 41st Millennium in pursuit of an unknown objective. In addition, the Warmaster of Chaos Abaddon the Despoiler and one of his lieutenants have discovered, through their interactions with the daemonic spirits of the Warp, that there is a "New Kind of Death" walking on Mars.
The idea of the Void Dragon slumbering beneath Mars was also given credence during the Horus Heresy. At the outbreak of the Schism of Mars in the early 31st Millennium, it was known by the Master Adepts of the Mechanicum who sided with Horus that the "Dragon of Mars," an ancient entity believed to have been defeated and captured by the man who would become the Emperor of Mankind before the start of the Age of Strife and the collapse of the ancient human galactic confederation, was hidden in the Noctis Labrynthus. A Guardian of the Dragon, a Terran girl named Dalia Cythera, was also interred within to keep others away from the entity and to keep the creature secured there. While the Void Dragon was not mentioned by name, it is pertinent given the fact that it provides significant evidence as to the possible location of one of the final two sleeping C'tan in the galaxy -- and this dreadful entity lies at the very heart of the Imperium of Man.
In 898.M41, the Necron Arotepk Dynasty assaulted the Eldar Maiden World of Silentia in pursuit of a precious gem that its lords believed was likely a C'tan Shard of the Void Dragon. In 912.M41, this Shard escaped their control. Though it possessed only a small fraction of the Void Dragons full power, it proceeded to lay waste to the Arotepk Dynasty's holdings and gorge itself on a dozen Necron worlds. Eventually, however, Arotepk Crypteks were able to force the creature back into its cage.