"An icon will fall. A god will awaken. A primarch will rise. The galaxy will burn."
- – Teaser trailer of Battlefleet Gothic Armada II, basically summarizing the whole Gathering Storm.
The Gathering Storm is a set of campaigns for Warhammer 40k, designed to advance the storyline after decades of stagnation. They are the last major campaigns for the 7th Edition, and the first introduction to the 8th edition.
The story around it
A series of events cause the status quo of the galaxy to be threatened, after millenia of stagnation. On one hand, Abaddon the Despoiler, after many centuries of planning, launches its 13th Black Crusade to try and destroy the Imperium once and for all. Unlike the other many times he had tried that, however, this time he actually gets something done, destroying the fortress world of Cadia, one of the few things that could stop the growth of the Eye of Terror. The unlikely alliances made by the Imperium, Eldar and the Necron lord Trazyn are unable to protect the planet. The pylons are destroyed, Cadia is exterminated after Abaddon just said Fuck it and crashed one of his Blackstone Fortresses into the planet's surface, Creed is apparently dead (but not really, though), and all of this starts a chain reaction that causes the Eye of Terror to expand, creating the biggest warpstorm there's ever been, the Cicatrix Maledictum.
While all of this is going on, the Eldar Farseer Eldrad decides that the situation of the Eldar is too worrying, and decides to start a ritual to bring Ynnead, the Eldar god of death, into being. This ends up pissing off everyone, most Eldar considering this as blasphemy (even HERESY), while the Inquisition don't trust the Eldar already. The summoning ritual gets interrupted by forces of the Deathwatch and only a small fragment of Ynnead comes to the material realm. This being reveals itself to a Yvrainne, a gladiator fighting in the arena of Commorragh. She heard the voice of Ynnead, and was transformed into its herald.
Unfortunately, this not only pissed everyone in Commorragh, but it also caused the Gates of Khaine to weaken, allowing daemons to enter into the realm. They manage to fight their way out of the Dark City, and with the help of the harlequins, end up in Biel-Tan. Once there, Yvrainne starts to reach about Ynnead, telling the Eldar there that they don't have to all die to defeat She Who Thirsts. This causes more confusion and fights between the factions, and in the heat of it all, a Slaaneshi force attacks the craftworld, shattering it and fracturing it. They are saved by the formation of a physical avatar for Ynnead, the Yncarne, but now the craftworld is damaged and losing population fast. Yvraine's faction, now called the Ynnari, escape from there.
The Ynnari rescue the remnants of the fleeing Cadian forces, now called the Celestinian Crusade in honor of Saint Celestine, at the ice moon of Klaisus, just as Abaddon's forces were closing in. The combined warhost drives off the Despoiler's forces, and a temporary alliance is forged. As one, it re-enters the Webway, with the aim of reaching the realm of Ultramar.
And why? To wake up Roboute Guilliman, Primarch of the Ultramarines, so he could serve as a figure that the forces of the Imperium can rally around, if it is to have any chance of surviving the upsurge in Chaos activity brought about by the Great Rift.
Despite Ultramar already besieged by Chaos Legionnaires, the Celestinian Crusade is able to succeed at its goal, and at the climax of a pitched battle right inside the Hall of Correction, Roboute Guilliman is resurrected. The Primarch proceeds to lead the remnants of the Crusade as well as the Ultramarines in first driving off the invaders from the Fortress of Hera, then from the planet itself. After the arrival of various Imperial reinforcements, Guilliman leads a counter-attack that relieves the other core worlds of Ultramar from enemy attack.
With the situation at Ultramar stabilized, the Primarch then leads a force towards Terra. Now called the Terran Crusade, it is waylaid by Red Corsairs and daemons led by Kairos Fateweaver. Imprisoned on a Blackstone Fortress, the crusade is freed by the mysterious Cypher and his Harlequin allies. The rescued crusade escapes into the Webway, but it is intercepted by the Thousand Sons. Led by the daemon primarch Magnus the Red, the battle spills onto the barren surface of Luna. Though Magnus has the upper hand, the arrival of Adeptus Custodes and the Sisters of Silence quickly even the odds in Guilliman's favor. Magnus is soon driven back into the Webway, and the battered and weary remnants of the crusade are escorted down to the Imperial Palace on Terra.
There Guilliman is allowed audience with his Father. When he finally re-emerged, and after driving off an attack of daemons upon the Palace itself, Guilliman announced the start of a new Crusade to stabilize the Imperium. The Indomitus Crusade had begun, and woe to all that would stand in the Avenging Son's way.
What this means to the 40k universe
The Gathering Storm events are the biggest shakeup of the status quo of the setting in a long time. Considering than in three books we've seen Cadia blown up, the Eye of Terror growing into massive proportions, an Eldar god being born, Commorragh being assaulted by daemons, a craftworld being almost destroyed, a loyalist Primarch being resurrected, a fight on the Moon between Guilliman and Magnus, the launch of a new crusade and the unveiling of a new breed of Space marine, we could safely say we're going somewhere now. This obviously brings forth the 8th edition, which creates its own changes in the crunch. Whilst the timeline has advanced to what we would think of as M42, a part of Guilliman's research revealed that the current timeline was incorrect, with a possible discrepancy of a thousand years. So it still count as 40,000.
Works around The Gathering Storm
- Fall of Cadia: Abaddon ragequits and throws his ship at Cadia, breaking the planet and opening the Great Rift. While the original plan was this rift to drive straight into Terra, things went not as planned and it ended up dividing the Imperium in half instead. He still considers it a win since, you know, half the Imperium is cut off from the other and in constant risk of becoming daemon food.
- Death Masque: Eldrad tries to wake up Ynnead, and gets interrupted by the Inquisition. Kind-of succeeds, and was able to bring Ynnead into existence even if it's only in a weakened form. Slaanesh is livid, while Nurgle and Khorne are just "meh".
- Fracture of Biel-Tan: Yvraine gets recruited by Ynnead, escapes from Commorragh and drives off a daemon incursion at Biel-Tan at the expense of severely damaging the Craftworld. Biel-Tan are now made of nomadic bands of survivors, and the growing group of Yvraine's followers prepares a plan to meet with the remaining Imperial forces who got out of Cadia.
- Rise of the Primarch: Yvraine, Cawl and Co. manage to wake up Guilliman, then following much angsting the primarch has a talk with his red brother at Luna. The Chaos Gods are currently losing their shit over Guilliman being resurrected. Fulgrim and Mortarion are also less than thrilled.
- Devastation of Baal: Hive Fleet Leviathan vs the Blood Angels and most of it's successor chapters. The fighting eventually devolves into a triple-threat match with the Sons of Sanguinius, Leviathan, and a Khornate warband led by Ka'Bandha. Leviathan is repulsed, at the cost of numerous Blood Angel and successor losses, while Ka'Bandha fucks off after cleansing one of Baal's moons of life.
- Dark Imperium: With the end of the Indomitus Crusade, Guilliman has to deal with a ton of things in Ultramar and the Imperium as a whole. About half the book's set in the perspective of a group of Unnumbered Sons, with their leader who'd eventually become a lot bigger deal than he'd anticipated.
- Dark Imperium: Plague War: The sequel to Dark Imperium, it continues the campaigns of the Plague Wars. Guilliman and Mortarion face each other, but most of the story revolves around a mysterious "saint" that might/might not be the Emperor. A lot of discussion about faith and the divinity of the Emperor.
- Battlefleet Gothic Armada II: The first vidya set up in the 8th edition version of 40k.