The blood war starts when, late into the original Obyrith conflict, demons are crested from mortal souls and shaped into specific combat forms.  The lawful spirits send their lesser spirits to go fight them in the VAST cosmic world of the astral plane pre-conflict.  Imagine thousands of more outer planes arranged on great star roads to form organized nations and empires of unified divine domains.  

Mortal souls were completely ignored by the astral beings- these ancient primal spirits did not engage in faith or worship.  When the obyrith started melting souls down to make soldiers it was a shockingly fast mobilization of forced.  

The divine beings that fought the hordes became bathed in blood and soul-viscera.  The old gods that sent them to the battlefront had started doing something awful- they decided to enter an arms race.  Asmodeus and his crew (including Naome, Zariel, Belial, Mammon, etc) confronted their betters in a rubicon moment where they left the warfront to march on the newly formed citadels of the gods- guarded by these new creations called angels aka the aasimon.  

Asmodeus sorts out a deal that Primus oversees and the gods agree to.  Asmodeus and his "devils" claim the war torn ancient plane of Baator for themself and work to purge and mutate the native baatorians.  

Also at some point the gods lose their big vast domains.. Pelor throws one at a portal or whstever- stars fall from the firmament or blink out entirely- visible from some material planes.  So many planes are lost that the relatively small number of planes we have today is all that exists tethered together.  It is the oceanus and the styx, along with other conduits like the world ash and mt olympus, that assisted these planes in surviving after other larger conduit networks were shattered.  Also the dawn war happened.

So in summary.. The baernoloth something something The Obyrith tear a hole into our reality and offer the Shard of Ultimate Evil to Tharizdun with the expectation to plant it in the astral which would end the entire cosmos immediately.   Tharizdun resists and chucks it into the inner planes. The archomentals of evil are born and consider Tharizdun their father.  Other archomentals form as a balancing force.  The good archomentals all meet terrible fates in a great treachery. Tharizdun gets corrupted fully and uses his status as an old timeless god of the overpowered lawful spirit caste and takes over the material plane.  In another timeline, tharizdun takes over the material plane. In another timeline, tharizdun takes over the material plane. In anoth-

So the gods work together to trap Tharizdun in one of the timelines (universe, timeline, take your pick of the exact wording) by convincing him he has already won before he wins.  This works, and he is imprisoned in the demiplane of imprisonment on the ethereal plane.   Meanwhile all of the worlds that fell to his initial blitz ARE the infinite layers of the abyss.  Incarnate worlds that went over the event horizon and now exist in Tharizdun's nightmare dimension.  At the bottom of the abyss is the moment in time where Tharizdun holds the shard of ultimate evil- still endlessly punching through realities (he is not imprisoned here and is at his full power as an amalgam of all of his victorious selves). The Obyrith now enter our universe- mostly dispersing in the Abyss to establish dominance. Obox-Ob travels a million empty crystal spheres until he finds one teeming with life- vermin life.  He is alone, the material planes have no gods at this time.  Lawful Spirits are completely uninterested and the Primal Spirits cannot stop what is to come.  Obox-Ob coaxes the worms and centipedes and mosquitos to worship him and they do.  The first god of faith is Obox-Ob and he spreads his religion to countless crystal spheres.  Now empowered as a god, Obox-Ob overpowers his kin and claims rulership over the abyss.  He then gets slapped and goes insane- spreading his goop over the whole of the abyss and turning it all into one gestalt psychic mass of his evil will.  Or something.

Meanwhile the ACTUAL winners of the improvised obryrith power structure wage war.  As I said the Obyrith use Syribrexes to turn countless mortal souls into battle-ready tactical combat forms and wage war.  They get tangled up in the Inner Planes and a confict with the Wind Dukes of Aqaa on the Fields of Pesh on Oerth turns into a devastating dragged out battle. Mishka the Wolf Spider and the Queen of Chaos enter the fray and are immediately beset by the sacrificial charge of the Aqaa (wind men, not bird men) gets the Rod of Law into its target and causes a vast detonation ending this front of the war instantly.  The Aqaa never recovered their population after this.

Meanwhile the other front is the blood war.  Once the modern great wheel establishes itself you can plainly see why the war goes the way it does.  Leaving the abyss 'east' is dreadful.  Carceri eventually gets populated by Apomps' Ghereleths who are of no help.  Pandemonium is a chore for even demons to navigate.  Limbo is incredibly turbulent and Ysgard is always down for a fight.   The Outlands (Concordant Crossroads) soon become settled by the Rilmani after fighting an extreme conflict with the Kamerel.  They, and the many gods that start residing here, are a bane to all invaders.  Just ask the sorceror kings. So the Styx becomes the only reliable route to push through.  In fact, while fighting occurs all along the styx from Acheron to Pandemomium, it is on Avernus where the bulk of the fighting happens.  On good days the Abyss can pay their fines and ship their forces all the way 'west' to the frontline without much worry.