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 of course 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 both 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 attack 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 sizedfuckoff hueg skulls). 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.
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 its a lost cause, has 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 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 its 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.