The Imperial Guard were having their asses handed to them by the Orks on the planet Ghorala in the Octarius system. Former Lord Inquisitor Kryptman, leading expert on the Tyranids and prolific bastard, decided that the best solution would be to sic a recently captured swarm of Tyranids on the Orks, let the two fight it out, and mop up the remaining survivors with the IG. The obvious flaw in his plan (besides it being a fucking nursery for one of these) is that both Nids and Orks are unwitting experts at bioengineering who thrive on war, meaning whoever ultimately wins will emerge much stronger than ever before (though hopefully if the Tyranids win they will be overspecialized Ork killers and be weak to other threats and fighting styles). The conflict has predictably devolved into a massive meat grinder, where neither side has a clear shot at winning, and the orks are fucking loving it.
Never mind on the Eldar part, they're currently trying to clean up Kryptman's mess by bio-purging the front line and the worlds around it. Unfortunately, they missed the Overfiend of Octarius, who just managed to loot an Avatar of Khaine... fucccckkkkk.
Oh, and Ghazghkull himself is on his way there too so he can call up more boyz for Da Great WAAAAGH!!!! Score one for the Orks, it looks like. After arriving there and killing a Mawloc from the inside out (eat your heart out, Alien), he's managed to take command of the Octarian Orks.
With so much carnage and destruction going on, it was bound to attract the attention of the Ruinous Powers. In particular, a Khornate warband called the Skullhunt of Vodha Bloodprice attacked the system, declaring a Blood Crusade. And their results are... mediocre, to say the least. In the CSM 8th ed. Codex, it says the khornates were able to offer "over
ni-eight thousand skulls to the Blood God, the smallest of which is the size of a boulder" and that was enough to make Vodha Bloodprice ascend to Daemonhood. It seems small, but Vodha did managed to off a Hierophant Bio-Titan with a Warboss' salvaged axe, so lords know what crazy misadventures he got himself into to merit princedom with a relatively small body-count (could've just been because they were 8,000 unusually sized fuckoff huge skulls; the phrasing also implies that any skulls smaller than a boulder were left out of the offering, meaning they probably killed many more and offered only the best to Khorne).
They fucked off right after that, probably because as they'd already achieved their objective and the Indomitus Crusade was underway, creating fresher pastures to drown in blood.
Since then, the dumpster fire keeps growing. Ghazghkull rallied the Orks and repulsed most of the Tyranids from the system and is currently getting them ready for what he's planning as the biggest, baddest WAAAGH in history. The Tyranids, realizing that it's a lost cause, have broken off from Octarius and attacking nearby worlds instead. At the end of the conflict, Kryptmann did two things. At best he bought the Imperium some time as the Tyranids are reeling from the conflict and the Orks are still gearing up, so there's still time to prepare for them. At worst, he just replaced the Tyranids with the Orks, as now the Orks are getting organized for a WAAAGH that could very well eclipse the devastation of the Tyranids once it gets started.
Now this brings up the question of 'Why not blow the planets around the war to shit? Why not just carpet-bomb the Octarius system with Cyclonic Torpedoes and Virus Bombs? Considering Ghazghkull managed to largely eliminate the nids in Octarius, that would mean he brought a naval fleet powerful enough to rival a Tyranid Hive fleet, something the Imperium struggles to scrape up in the best of times. So if an Exterminatus fleet were to try and purge Octarius itself, they'd have to contend with splinter fleets and their Shadow of the Warp interference AND the bulk of a massive WAAAGH-ready naval fleet at the same time. This would take resources the Imperium currently can't afford to spare given how Chaos is currently banging on their battle-worn doors and Papa Smurf is too busy doing everything to keep them from breaking it down.
Basically, it's a lose, lose, lose situation for the Imperium. The only solution seems to revolve around buying time and cordoning off as much territory as possible---Whatever spews forth will essentially be warp tainted Ork/Tyranid Hybrids led by Ghazzy... Seems like they have started fucking off to nearby planets after the Great Rift was formed. Though most likely nothing will come of it, since GW decided to advance the plot a couple hundred years and the only way they can resolve the whole kerfuffle without fucking over their most popular factions is to quietly sweep it under the rug (having the Rift eat it would serve the purpose well)
At a Warhammer reveal event the second 9th Edition Warzone series, Warzone: Octarius, was revealed. The plot follows Imperial Guard, Inquisition, and Deathwatch in Ocatrius. Kryptman's "genius" plan has led to a new threat where the Orks and Tyranids could spill out their cage fight outside the cage. As such Imperial Forces are trying to contain the threat which could fuck up so much shit. Also involved is the 9E Kill Team, which had its first boxset set in Octarius, where plastic Kriegers with shovels fought against a band of Kommandos dressed like full-on Ramborks.
The Black Templars are also en route to try and help fix the situation, and if the rumors are true, the Imperial Guard will be deploying there in significant numbers as well in their Codex.
The Stalemate Breaks
While the Orks and Nids were off killing each other, Inquisitor Nashir Sahansun created a planetary cordon around the major warzones of Octarius to cut off any venues of escape. To this end he convinced space marine captains, planetary governors and navy admirals alike to keep watch around these barrier planets, making sure to prune off any branching warbands that managed to escape their war. While he was sure that whatever would break out would inevitably be far worse than what came in, he was hopeful that the forces maintaining the cordon would be experienced enough and proficiently armed to fight back the victors. Unfortunately, there was no winner; the warzone merely widened, with Orks and Tyranids claiming systems where they stood and Sahansun found himself cursing Kryptman's name further.
But then a Deathwatch kill team made a discovery: A random Ork mek somehow managed to build a device on his head that managed to nullify the effects of the shadow in the warp typical of the Tyranid hive mind. Inquisitor Athocles Van Roth took extreme interest in this and performed extensive experiments on this device, thinking it to be the final solution to the Tyranid menace. Once satisfied, he went to the Maiden World of Veloria, a place where the fighting was relatively minimal so he could perform his full-scale testing. Unfortunately, any attempts to replicate the device failed, because ork technology is nonsense and any attempt to graft the original device onto another living being only worked if said being was another mek... and then more Orks came. Apparently, they had been drawn by that weird head-rig at a rate too quick to ensure the proper withdrawal of his Imperial forces. Luck would find him again when he found a rival inquisitor who warned him that they would need the help of the few surviving Eldar of this world to destroy their own tower - a task the craftworlders agreed to and would accomplish at a great cost to both the inquisitors and the Eldar. Even this would prove to be a futile effort however, as the Deathwatch team tasked with watching over the device was cut down to a man and any manner of aerial support was lost to him.
If there were any hope that they could possibly break free of this world, it would still mean little as the experiment turned out to be a colossal failure.
The Consumption of Octarius
The Black Templars and their accompanying reinforcement arrived at the battered but vital Forge World of Sigma-Ulstari just in time to save it from the latest wave of Tyranids, only to immediately get attacked by a new group of boyz led by Warboss Tuskagrob Wurldkilla. High Marshal Helbrecht led the defense, eventually leading a boarding action onto one of Wurldkilla's (unshielded, for some reason) squiggoths pulling one of six explosive battering rams towards the Imperial walls at Alpha-primaros. Of course, he picked the biggest, strongest one in the lead, which meant he picked a fight with the Warboss himself, and preceded to get his ass kicked. The Marshal was driven to one knee, his remaining organic arm shattered and skull cracked. Just as the Ork was about to go for the killing blow, Helbrecht shot him in the knee with his combi-melta, then shoved a sword in his head while he was down. Cheap shot or not, between the Black Templar's efforts and the successful boarding action on the squiggoths, the back of this Ork incursion was broken.
The local Mechanicus was still fucking furious though, because none of the aid that they'd been promised had come from Inquisitor Sahansun. The Templars and friends just happened to be in the neighborhood for unrelated reasons. They promptly decided the only reasonable thing to do was cancel all their promised aid and weapons to the surrounding worlds and keep it all for themselves, sentencing themselves to total isolation in a sector now doomed to fall in short order. That'll show 'em!
Meanwhile, on Octaria itself, big things were happening. The Swarmlord was on the Ork's capital world, leading the charge against the Overfiend of Octarius, when all of a sudden Khorne's Blood Crusade rolled around, hitting both sides with waves of daemons, Chaos Marines, and cultists. The fighting was brief but incredibly intense, and with the daemons offering little of sustenance to the Tyranids, singularly unprofitable. Meanwhile, the Orks reproduced just the same no matter who they were fighting. By the time Chaos had either vanished back into the Warp or been slain, the balance of power on the planet had shifted in favor of the greenskins. The Overfiend realized this and pressed his advantage, pushing the bugs back across the entire planet.
Realizing that it had to kill the Ork Warlord if he wanted to prevent a total Tyranid defeat on the planet and the boyz taking the WAAAGH on and shifting the entire tide of the war, the boss bug sought out his opposite number with a mob of Tyrant Guards in tow. But the problem was, by this point the war had been going on so hard and so long that even the Overfiend's Skarboyz were the size of Killa Kanz. It proved to be an intense slugging match that lasted for hours, but in the end the Overfiend managed to punch the Swarmlord in the midriff with one titanic Power Klaw, crush his head while he staggered, and then rip the leader-beast in half. Other synapse creatures moved in quickly to maintain control of the hordes and conduct a fighting retreat, but things on Octaria were looking grim for Hive Fleet Leviathan.
Or were they?
The Hive Mind wasn't stupid. Even while the fighting was raging on the Ork's capital, it had realized that it would need more biomass to win the war there. So, while its bugs and the boyz were busy slugging it out on the ground, it had dispatched portions of the fleet to conquer and consume lesser Ork-held worlds, including Ghorola, Keltor, and Derragon. While the formation of the Great Rift had delayed the arrival of these reinforcements, the gambit below had bought it enough time to move thousands of new bio-ships into the system. The Swarmlord respawned on the largest of these incoming reinforcement tendrils, hungrier than ever and ready for some payback.
Dispatching portions of their fleet to the surrounding planets to keep the Orks there distracted and in no position (or mood) to reinforce Octaria, the Tyranids launched their latest orbital assault, reinforcing the bugs still on the planet with billions of fresh arrivals. Even cut off, though, the Overfiend's boyz had turned the entire planet into a deathtrap. Entire mountain ranges had been turned into weapons, and miles upon miles of rock opened up when Tyranid spores struck them, revealing vast pits full of buzzing, serrated teeth. Even the largest bio-titan falling into one of these traps left it little more than chunky salsa. The Swarmlord didn't care much about the losses though - all that mattered was consuming the planet, and it would get all the biomass back.
Once their landing zones were secure, the Tyranids started spawning their biggest beasts like there was no tomorrow. Carnifexes, Mawlocs, and bio-titans were thrown in battle like they were gaunts, split into two groups. Playing on innate Orkish aggression, they goaded the Overfiend with continuous waves of their finest specimens, gradually drawing him and his retinue out and away from their fortress while using the other half of the army to form a perimeter. The Overfiend and his boyz fought for hours, killing thousands of monsters but in the process dulling their blades and running dry on ammo. When the Warboss himself took a thigh wound in a Carnifex's death throes, the boss bug went in. This time, having worn down the enemy ahead of time, the Swarmlord parried the Ork's blows, cut the exposed cable on his Power Klaw, and finally ate the Overfiend's brain.
With their warlord dead and Ghazghkull nowhere to be found, the Orks in the Octarius sector broke apart, with at least six different Warbosses vying for the title of Overfiend. The Tyranids, fresh off consuming the Octarius system entirely, took advantage of that to push the war-hardened greenskins out by the billions, straight into the Imperial lines.
No telling yet exactly how the Imperium has reacted to Leviathan's victory, but it certainly ain't looking good for Inquisitor Sahansun. Meanwhile, Kryptmann still insists he was right.
Tl;dr: Nids won, Orks are running for it, Kryptman's an absolute dumbass, and shit's fucked.