I had a thought, with the Big E and three of the four Chaos gods on the same side, this might happen in the same vein that chaos undivided armies happened in the past when they attacked the Imperium:
Well this is awkward
"The forces of the warp had always been a capricious lot, forming and dissolving alliances almost at the drop of a hat, for such was the nature of chaos, and yet I never would have thought that such an alliance as this one would come to be," mussed ancient Rylanor as he fired his assault cannon at an entrenched position of the Death Guard, providing covering fire for a horde of World Eaters and Emperor's Children who all to eagerly charged the followers of the plague god.
"Join the Legion of the Damned, they said. You will get to kill traitors and avenge your fallen brothers, they said. Well I guess they never specified which traitors they meant," grumbled the burning dreadnought, as he moved to plug a breach in the front line where a horde of rusted demon engines had broken through and were now menacing a coven of Tzeentchian sorcerers acting as artillery support.
"Throne damn it all, this day can't get any worse." he thought, moments before a thick corrosive fog engulfed the battlefield, reducing visibility to about as far as his powerfist could reach, which was plenty as far as the ancient dreadnought was concerned.
So he threw himself into the melee, shattering rusty metal and pulping rotten flesh with unadulterated fury, pretending the sorceress he was protecting were sanctioned psykers and magnanimously refrained from stomping on the Hellscourges who came to support him.
Then a sudden humming caught his attention, the noise at first being drowned out by the sounds of battle but swiftly growing in volume. In sudden relation of what the sound meant, Rylanor raised his left arm and unleashed a stream of golden fire from the heavy flamer build into his power fist's gauntlet, catching a swarm of demonic insects that had used the fog as cover in his attack, sending most of them to the ground as burning corpses before they had a chance to enact their surprise attack.
Yet he failed to catch all of them, one in particular rushing him with its deadly stinger poised as it closed the distance with the source of the hateful fire, the fog hiding its approach from Rylanor's notice until it would have been too late. Yet, a different figure interceded, striking the insect out of the air with a single swordstroke.
"Good strike," complimented the dreadnought it's savior, having only glimpsed a vague outline of the figure through the still heavy fog, before refocusing his attention on a charging hellbrute, stunning the pitiful dreadnought thing with a powerful left hook before unleashing a barrage of bolter rounds with his right.
"Nicely done," complimented the figure back, as they cut down a score of poxwalkers that had been following the deceased dreadnought's charge.
Rylanor stilled at the words of the stranger, the voice sounding eerily familiar, before turning towards the figure just as an arcane wind emanated from the sorcerer coven behind them and dispelled the fog, restoring visibility. "Fulgrim!?" the ancient cried out, half in rage and half in horror.
Reacting to his name, Fulgrim turned back around as well, his usual cocky smile twisting into an expression of pure shock at who he was fighting alongside. Both warriors stood there frozen for a moment, the fighting around them swiftly dying down as the last of Nurgles's followers in the immediate area were dispatched and the gathered forces dispersed to rejoin the front line.
It was the demon prince, who regained his barring first. "This never happened!" he hissed.
"Agreed," answered Rylanor almost instantly, before both turned around and made to move in opposite directions.
"By the throne, nothing makes sense anymore," lamented the ancient, a sentiment firmly shared by his disowned gene-sire.