The blood war has waged for uncounted eons, since history began, and perhaps even before it. However, both the demons and devils know that the true prize is the prime material plane, and even if it takes an eon or two to arrange a meeting, it's still going to happen when you live forever.
So it was that a group of the highest ranking arch devils and demon princes met to hatch a fiendish plot. A gate would be built, but it would be no ordinary gate. This would transport a select cadre of both camps to the Prime, transmogrifying them as it did so that they would no longer be beholden to the rules for outsider travel, but would instead have unfettered access to that most prized location. This elite group would set up a base and begin the preparations for full scale invasion, and the rich harvest of souls could then begin. Perhaps even the blood war could be ended.
And so it was, another eon later, that the selected army of high ranking demons and devils were assembled before the gate that had been built by the mage smiths of both races, deep in Pluton, the lowest plane of Hades. Of course choosing the middle ground required the services of the Yugoloths to act as mediators, the price for which was their own contingent in the invading army.
Intoning horrible words of power, the gate was opened, and the army stepped through.
Needless to say, both sides were lying. End the blood war? Share the Prime with the hated enemy? Not bloody likely. Both powers were needed to make the gate work, but neither side had any intention of letting the other through. The devils, arch crafters of law, delved deep into their wellspring, and crafted a law lattice as an integral part of their half of the gate, against which the creatures of chaos would be smashed. The demons for their part, no slouches when it came to backstabbery, delved into their own lore and crafted the metaphysical equivalent of a thorny hedge, against which the creatures of law would be helpless, ripped asunder. Not only were the Yugoloths aware of the double treachery, they were involved in the creation of both lattice and hedge, and then of course spent time building warding spells that would ensure their contingent passed through both barriers, leaving the demons and devils behind
It goes without saying that the hedge and the lattice didn't sit well together, but instead merged into something truly terrifying. As soon as the army passed through the gate there was a titanic explosion. It not only killed all those in the Gate room, it wiped the entire lower plane of Hades from existence.
The elite cadre of Ta'anari, Baatezu and Yugoloths that entered the gate didn't fare too well either. Most were utterly annihilated by the hedge lattice construct. Only a very few were able to push past/through the construct to the Prime beyond, but each of those suffered horribly. The first things to go were their items, weapons and armour. Enough magical equipment to sate a god of avarice was stripped away and torn to shreds by the insatiable lattice hedge, and as those few creatures continued forward, it began to tear away at their essence. Eons of training, knowledge and power were torn away like so many leaves in a wind storm. By the end of it, those few that were left and managed to battle past the lattice hedge had regressed so far that they were, once again, eggs...Eggs though that quickly hatched, spilling their newly formed prime bound outsiders into the world...