There may be a connection between this being and the Dragon of Mars, an unknown entity which was imprisoned beneath the surface of Mars by the Perpetual who would become the Emperor of Mankind before the start of the Age of Strife.
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 Aeldari 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 Aeldari 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 the Aeldari myths, the Void Dragon was only defeated when the legendary artefacts known as the "Talismans of Vaul" were placed about the Void Dragon's neck. At that moment, a light spilled forth that revealed its secret weakness. The C'tan was ultimately defeated by the Necrons it had once enslaved and was shattered into shards.
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 Aeldari legends reviewed by Horst, these warriors could channel lightning into their foes, though these lightning attacks were ineffective against the "Iron Knights" that the Aeldari hastily created to battle against the ancient Necrons.
Having gained a respite through the forging of the Iron Knights, the Aeldari god of the forge, Vaul, created the "Talismans of Vaul" (Blackstone Fortresses). These too were powered by the souls of Aeldari Farseers and were used by Vaul to attack the Void Dragon directly.
Here the Aeldari 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 Aeldari attack had succeeded in stopping the Void Dragon's rampage, but ultimately had failed to destroy the Star God.
Using machineries and technologies conceived by the Void Dragon, the Necrons planned to turn the tide of the war by cutting off the Old Ones from the source of their psychic power: they would forever separate realspace from the Warp with an unbreakable barrier.
Protected by massive warfleets, the Necrons began to construct Warp suppression pylons at key locations across the galaxy, such as Cadia in the Segmentum Obscurus and the Cerberus Shroud in the Segmentum Tempestus. This plan was interrupted, however, and the arcane mechanisms were never fully completed.
In 912.M41, the Necron Arotepk Dynasty was ravaged by the mindless rage of a C'tan Shard of the Void Dragon that had escaped from its imprisonment. Though only a faint shadow of the Void Dragon's true power, it proceeded to gorge itself on a dozen Necron worlds, expending its fury upon the living before the Arotepk Crypteks were able to force the creature back into its cage.
Dragon of Mars
While only speculation at best, it is believed by experts knowledgeable on this topic that the Void Dragon may actually be on Mars, likely in the Noctis Labyrinth, where it is known to the Adeptus Mechanicus as the entity called the "Dragon of Mars."
This is believed because of the suicide rush in 998.M41 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 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 ancient 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 Age of Strife, was hidden in the Noctis Labrynthus.
A Guardian of the Dragon, a Terran girl named Dalia Cythera, was also transformed into a Perpetual and interred within to keep others away from the entity and to keep the creature secured there.
It unleashes this potent force as an energy that ripples throughout the necrodermis bodies of its C'tan Shards, creating a constantly shifting, digitally pixelated appearance for its physical forms -- as if they exist in some sort of quantum state that is in flux between realities.
- Dragon of Mars
- Codex: Necrons (3rd Edition), pg. 26
- Codex: Necrons (5th Edition), pp. 5-8, 40-41
- Codex: Necrons (7th Edition) (Digital Edition), "A New Epoch Begins"
- Codex: Necrons (9th Edition), pp. 26-27
- Mechanicum (Novel) by Graham McNeill
- Games Workshop Website - Dawn of the C'tan: Translations of pertinent Eldar mythic cycles - Part 3.
- Battlefleet Gothic Magazine 13, pg. 23
- Warhammer Community - A New Star (God) Rises