The World Engine
A gloriously epic and totally awesome fight between the Space Marines and the Necrons. This story is reminiscent of the glorious days of yore, when Space Marines were real heroes(Sons of Dorn) and when the enemy brought something new and ungodly to the table, the Imperium went balls-to-the-wall and risked total failure on a crazy awesome plan.
An untainted gem hidden in the annals of the 5th Edition Space Marine Codex, no sight of Wardian stupidity or Ultrahmahreen fanwank exist here. Only awesome. Only sons of Dorn. And now it's a Battles novel.
Tell me a story, Grandpa
The World Engine was a planet-sized Necron space vessel that set out when the Necron Lord of the Tomb World Borsis was usurped by Necronian politics (this was written in the 5th Ed Marine Codex, mind you!) and the revolutionaries, presumably led by Tut Guevara, took the WHOLE TOMB WORLD for a little joyride. They appeared in the Vidar Subsector at the close of M41, beginning their reign by Death-Starring the Agri Worlds of Gaios Prime and Gaios Tertio of all life, using the largest Gauss weapons known to man.
The Imperium got its act together uncharacteristically promptly, and sent the whole sector's fleet, and detachments from fifteen Space Marine chapters, including the Ultramarines, the Astral Knights, the Invaders, the Blood Angels and the Aurora Chapter to head off the World Engine at Safehold before the Necron punks could get any further.
Now, the main problem with the assault was that the Giant Ball of Haetraep had the Necron equivalent of Void Shields so powerful that not even that many ships could do anything to it. No torpedoes, no Drop Pods, not even Nova Cannons could touch the thing. It also blocked all teleportation attempts, killing the three full squads of Terminators the Invaders sent to infiltrate it. All the while, the Necrons happily blasted away at the Imperial fleet, destroying and crippling dozens of ships.
But then something wonderful happened. Chapter Master Artor Amhrad of the Astral Knights decided that he would be awesome today.
Chapter Master Artor Amhrad of the Astral Knights decided that if weapons can't break the shield, maybe something bigger would. He brought his battle barge, Tempestus, around to bear, fired up the engines, and rammed it directly into the World Engine.
And it totally worked. He sent the entire Astral Knights chapter to war, now that his ship was through the shield and crashed on the World Engine's surface. The Astral Knights proceeded to kick the collective buttocks of a world's worth of Necrons over a 100-hour period. They blew up everything they found in their path, yet it seemed not even that would be enough, however the Space Marines persisted; because Chapter Master Artor Amhrad had a plan.
Chapter Master Artor Amhrad was going to overload the WHOLE PLANET.
There are two versions of this final battle. Both are epic as fuck, so we're printing them both. In the first, only Artor Amhrad and five other Marines were left standing, while the surrounding fleet continued to pound on the still-impenetrable shielding while being slaughtered by the still-mostly-operable Gauss batteries. The team finally hit the Central Command Tomb after one hundred hours of continuous fighting. After presumably strangling Tut Guevara to death with his bare hands (how do you strangle a robot!? By popping its head off of course) and beating the sorry scrapheap's head in, Chapter Master Artor Amhrad planted meltabombs all over the Command Tomb, blowing it, himself, and his five remaining, finest marines to kingdom come. Because the Knights had destroyed so many generators and command nodes, when the Central Node exploded the power couldn't regulate itself fast enough, backed up, and blew out the whole shielding array, most of the remaining weapons, and several other critical systems in a glorious chain reaction.
In the second, Amrad sent all the survivors of the Chapter on a suicide distraction attack, meanwhile a crack commando unit made of the entire Astral Knights' HQ and lead by the man himself went to plant meltabombs at the central generator of Borsis in order to release the imprisoned C'tan who was powering up the entire World Engine, after fighting against some crazy-scary overpowered Necron Judicator (we really need a mini of him) Amhrad blew himself and every Necron and marine up at the generator, releasing the C'tan who proceeded to mulch evil Overlord
Tut Guevara Heqiroth out of existence along with most of the Engine's power systems.
Point is, no matter which version you read, it's fuckawesome final act of FUCK YOU meltabomb sacrifice. With his last act, Chapter Master Artor Amhrad had both ended the Astral Knights Chapter, and saved the Imperium from this unstoppable foe. Oh, and in the second version, the last Astral Knight, a librarian whose mission was to record everything in his mind so when the Inquisition found his corpse they would know what happened, vowed to the C'tan the Imperium would hunt him down too, because that's how imperials roll.
The after-mission report was similarly filled with win. The fleet remained in orbit for several weeks after, not because they were searching for any remaining Necrons, mind you. No, they were letting the Adeptus Mechanicus actually sift through the wreckage for useful stuff. And, the Ultramarines salvaged the wreck of the Tempestus themselves, towing it down to make planetfall on the recently-harvested Dead World Safehold, to raise an Imperial Shrine inside the wreck dedicated to each of the last 772 Marines of the Astral Knights Chapter who fought and died to cripple the World Engine. Every one of them got a personal statue inside the shrine. And despite the fact that the world is totally deserted save for only about nine scavengers that still live on the entire planet to this day, the shrine is personally guarded by volunteers from each of the 14 Space Marine chapters that fought alongside the Astral Knights. Fucking. Metal.
Either inspired from their example, or Gee Dubs wanting to give more justice to this: this event re-occurs in the officially-licensed video game: Battlefleet Gothic Armada 2, against a Necron vessel of similar size called the "Dark Throne", although under different circumstances. Shadow Captain Ajaz Solari of the Raven Guard used his Battle Barge as an oversized drop pod, as it become impossible to land Thunderhawks or drop pods on it's surface, and sacrificed himself and his company in order to destroy a Necros planet-sized weapons platform by using a vortex bomb.
You, as High Admiral Spire of the Imperial Navy, must protect Solari from the incoming Necron fleet as he desperately attempts to plant the bomb inside the vessel and bring it to heel.
Alternatively if you play the Necron campaign, you will play as Phaeron Amarkun of the Nepheru dynasty and be tasked with preventing this from happening, as the Dark Throne is actually a massive anti-warp device similar to the Cadian pylons, although much more defensible as it's also a planet-sized fortress on par with the World Engine.
Imperial Historians in-universe speculate that Port Maw, an artificial planet discovered during The Great Crusade and the largest Imperium star port in the Gothic Sector, was a botched or abandoned Necron world engine project. Currently they don't have any real evidence for the theory besides the fact that the area is filled with other old Necron artifacts such as the Cadian pylons and the blackstone fortresses.