Mag'ladroth - The Void Dragon

The Void Dragon is believed to be one of the four remaining C'tan, the Necron's Star Gods, in the Milky Way Galaxy. The Void Dragon, if it actually exists, is arguably the most powerful 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 years ago, although the Nightbringer seems to have had a much more sinister effect on the galaxy as a whole. The Void Dragon 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 xeno 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 knowledgable 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 addition, the Chaos Space Marines' Warmaster Abaddon and one of his lieutenants have discovered, through their interactions with the 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. During the the outbreak of the Schism of Mars in the early 31st Millennium, it was known by the Master Adepts of the Mechanicus who sided with Horus that the "Dragon of Mars" was hidden in the Noctis Labrynthus, with a Guardian of the Dragon 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.

http://warhammer40k.wikia.com/wiki/Void_Dragon