Octarius War
The Octarius War is another recent hilarious and fail venture the Imperium of Man has made, though unusually the most catastrophic part was when the Imperium tried to not fight anymore.
Overview
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.
This tale is notable for being one of the few bits of fluff in the Nid 'dex where they don't get their arses handed to them by space communists, space elves, or fucking smurfs.
Team Deathmatch!
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 hueg 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 then offered only the best to Khorne). They fucked off right after that, likely as they've already achieved their objective and the Indomitus Crusade was underway, so there were fresher pastures to drown in blood.
More Recently
Since then, the worst-case scenario has gotten even worse. 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 be essentially be warp tainted Ork/Tyranid Hyrbids 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 aka the Octarius Sector. 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 Kreigers 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 Kryptmann's name further.
But then a Deathwatch kill team made a discovery: Some Ork mek just 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, and the attempts to graft the original device onto another living being only worked if that being was another mek...and then more Orks came, seemingly 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.