The entire WH40K galaxy VS Karma Abyss(Mom please don't come adventuring with me!)
Turns out the Dragon of Mars wasn't Magladroth, but Karma. Also turns out that The Emperor is really the reincarnation of Ryuu, who is the reincarnation of Jud, the greatest hero of the long-forgotten past. And that the "fight" between them was simply an over-protective mother making sure that her only son was strong enough to take care of himself before sending him out into the world.
Well The Emperor is now stuck on his throne, the Necrons are waking up, gathering their strength, and racing the Tyranids to the interior of Ryuu's kingdom.
What is Archmagos Cawl to do to save mankind, but to call upon the darkest and best-kept secret in the Imperium and awaken the one who has slept for since time immemorial: The Emperor's overprotective mother!