War of Beasts
The War of Beasts is the term used to label the ongoing conflict between the forces of the Imperium of Mankind and the various Xenos and Chaos forces trying to wrest its control of the strategic planet of Vigilus. The loss of Vigilus would mean the loss of a vital staging area for the Imperium for any kind of crusade into Imperium Nihilus, and as such it is important for it to stay in Imperial hands.
The Opening Phases
The Speed Freekz Cometh
The first major threat to hit the planet in the wake of the Great Rift opening was a massive Waaagh! of Orks. Bursting from the Rift seemingly at random, and led by powerful Speed Freeks, the Ork fleet easily broke through the naval cordon and landed its boyz in the deserts between the great hives.
While most of the fleet crashed and burned, the Orks soon used remains of the downed fleet to create ramshackle scrap-cities, and from there Speedboss Krooldakka roared out at the head of a massive force of bikes, trukks, and battlewagons. Still reeling from the birth of the Rift, the Imperial forces gathered could do nothing to stop it.
At first, the Orks met little success, as the force fields were too strong. Growing bored, they resorted to raiding caravans or battling and racing each other in the massive deserts outside of the hives. This state of affairs didn't last however; the Great Rift continued to grow, and with its expansion Warp Storms washed over Vigilus.
The bastion fields soon flickered, then failed outright, which was what the Orks were waiting for. The green tide rushed to each Hive, and despite the best efforts of the Imperial Guard, the Skitarii of the Adeptus Mechanicus, and the Sisters of Battle, it was all they could do to avoid being overwhelmed.
They Come From Underneath
Despite the threat of the Orks, there were threats to the defense of Vigilus much more closer to home. Centuries before, a Genestealer Cult called the Pauper Princes infiltrated the underbelly of many of the hives in Vigilus. Led by the Genestealer Patriarch called the Grandsire Wurm, it refused to let its years of preparation go to waste due to the unexpected greenskin arrivals. Uprisings that were supposed to be triggered decades down the road were forced to do so prematurely, and soon every Hivesprawl was in chaos due to the simultaneous insurrections. Imperial defenders fell to synchronized ambushes across the planet, as the Cult consolidated its hold on the Hives.
The response to the Genestealer Cult-led revolt was slow, as the Imperial Government initially thought it was the work of criminals and malcontents, and instead focused on shoring up its defenses. These uprisings, plus the constant ork raids, essentially forced the industry on the planet to grind to a halt. Things were, to put it mildly, looking grim.
Tooth and Claw
It was at this point that a task force of Space Marines made of elements from the Iron Hands and the Brazen Claws, decided to throw their hats into the proceedings. On the way to the Stygies Front, they made a detour to Vigilus once contacted by their Mechanicus contacts. Led by Kardan Stronos, the Astartes made a massed drop pod assault on the outskirts of each major Hivesprawl, before converging toward Megaborealis. Expecting to find Orks, they found cultists and Genestealers instead. Battles with these cultists would keep the sons of Ferrus Manus busy for many months.
Later on, a second task force of Astartes, this time Space Wolves from the Great Company of Ragnar Blackmane arrived to do battle with the xenos occupying the Oteck Hivesprawl. There was little coordination between the Wolves and the Iron Hands, and hilariously none of these Chapters saw fit to inform the Vigilus ruling council of what the heck were doing in their cities.
The Megaborealis Throwdown
A Mechanicus stronghold, it was thought unassailable by Ipluvius XIV, the Magus Dominus in command of the Skitarii forces there, enough that he let himself get distracted by his studies. However, a squad of Ork Kommandos begged to disagree, and brought down the hive's forcefield. The massive speed freekz attack that followed, coupled with a simultaneous attack by genestealer cultists, caused the magos' cogitators to freeze up due to the stress, leaving the defenders without coordination. It was bad enough that the cultists were able to steal Vortex Missiles and detonate it in the middle of the Hivesprawl, destroying countless Imperial war assets (including the Warlord Titan Dominus Rex).
In a decisive move, Archmagos Nesium Caldrike took control of the remaining defenders, and coordinating with elements of the Iron Hands Clan Kaargul put a stop to the Genestealer shenanigans, for now at least.
While all of this was happening, the Imperial Guard was making a large-scale counterattack, with the aim of reinforcing Megaborealis. The armored convoys of Leman Russes and Chimeras took a heavy toll on the greenskins, but all this just seemed to draw more and more mobs of speed freekz to the convoys.
Chaos Crashes The Party (Literally)
At Hivesprawl Dontoria, already under pressure from mysterious attackers (who turned out to be Dark Eldar opportunists), an unknown spacecraft decided to crash into the hive's center, despite the best efforts from the anti-air emplacements surrounding it. From its ruined depths came the mutants of the Gellerpox Infected, but the worse was to come, as in their wake came the Heretic Astartes of the Death Guard.
While the Tempestus Scions there were able to repel the Infected's assault, the Death Guard soon overwhelmed the defenses. In desperation the Hive's lower levels were firebombed in a two-mile radius, but this was not enough to contain Nurgle's followers, and soon a strange plague spread through Dontoria.
Here Come Guilliman's Boys
After receiving a vision from Tigurius, as well as a message from Roboute Guilliman to not allow Vigilus to fall, Chapter Master Marneus Calgar rushes as many of the Ultramarines that he could spare to the planet's defense. The going was difficult, and even with Tigurius' guidance many Epistolaries die to get the Chapter to the world on time.
Once there, he swiftly has those in the Aquilarian Council he senses to be in-cahoots with the Genestealer Cults executed, and has the Vigilus Senate set up in its place.
Wolf Bites Bug
The Space Wolves of course, were not idle. At Oteck Hivesprawl they tried to reclaim the all-important water supply back from the cultists. Things turned nasty very quickly when the Wolves "allies" in Guard turned out to have been controlled by the Cultists, and a whole lot of Hive City was suddenly dropped on the Wolves' heads. Many died, but enough Battle Brothers remained to continue the hunt for the xenos. The cultists were finally cornered and annihilated, but in their wake they left the water supply mostly tainted.
Haldor Icepelt, the commander of the Space Wolves there, ordered whatever explosives the cultists stockpiled found, and then used these to bury the polluted water source under tons of rubble, preventing its spread to other hives.
The Conflict Escalates even MORE
Eldar Can't Leave Well-Enough Alone
It was at this point that a Farseer from Craftworld Saim-Hann convinced that it was important for its forces to intervene in the conflict, as something there would influence the future of the Eldar race. Apparently a home-grown Tzeentchian cult planned to trigger a magma explosion that would eventually wipe out the leaders of Vigilus' defense, which would lead to the loss of the planet to Chaos, so they can't have that now, can they? Amazingly, one of the times the Eldar had genuinely good intentions (for themselves like usual, but in a way that honestly benefits someone else).
Using an underground Webway gate, the Saim-Hann task force, led by Autarch Rhyloor and Spiritseer Quelanaris, swiftly isolated the Chaos cultists and slaughtered them to a man. Word of their arrival spread to the Imperial Guard stormtroopers stationed nearby however, and the Imperials rushed to the location, fully convinced that these were the same Eldar who had been gleefully raiding the Hivesprawls during the past few months (and the Imperium does not distinguish between types of Eldar since the only difference that matters is that one takes captives and the other doesn't).
Despite Quelanaris trying desperately to explain to the Guard that, no, they looked nothing like the Dark Eldar dicks who were the cause of all the trouble (while standing over a bunch of Imperial corpses the soldiers have no reason to believe were guilty of anything), the Imperials were unconvinced (by the species that lies and manipulates constantly and has almost never been known to tell the truth) and, not surprisingly, opened fire. Many Eldar were cut down, including the Rhyloor, but Quelanaris managed to escape with her fallen comrades' spirit stones. Vowing vengeance (even though it was all her fault), she had the spirit stones installed into waiting Wraith Constructs, all in preparation for Saim-Hann's eventual return to Vigilus. Which, of course, will not only get more of her Eldar killed but will also justify the Imperium finally getting pissed enough to wipe Saim-Hann out of existence due to the extreme importance of Vigilus.
The Eldar habit of constant lying has come back to bite them yet again.
The Carnival of Pain
While the Imperial Guard wasted their time with the Craftworlders, the actual Dark Eldar continued their merry time harassing and generally making the residents of Hivesprawls Hyperia and Oteck wish for the swift release of death. Led primarily by the Haemonculi coven of The Altered, Kabalite troops joined with Wych cults and Haemonculi constructs in raids that were both spectacular and deadly. The Altered in particular had a specific beef with the Ultramarines, as Guilliman had kicked their ass in during the Indomitus Crusade, and took particular amusement in hauling entire squads of Ultramarines back to Commorragh for "entertainment". Though, the sheer casualties this probably cost him will no doubt make him question whether it was worth it. Revenge is all well and good but if you don't have many subordinates left from the process in a culture that would gleefully turn you into a torture flower for being vulnerable...
Holding Back The Foul Tide
Back in Dontoria Hive, the newly-arrived Primaris Chapter Necropolis Hawks fought to hold back the Gellerpox Infected, while the Crimson Fists and the Iron Hands worked to create a barrier to hold back the Chaos-spawned plague. Using the PDF's hellhound tanks, the Fists set fire to the hive's promethium pipelines, creating a literal firewall around the infected zone.