Was he
#1 a primordial horror sealed away by the Protogenoi before the Outer Planes were really formed?
or
#2 something present throughout most of the ancient Law/Chaos War and vanquished by the combined might of the new Gods of the mortal Races.
Existing canon seems a bit contradictory about this - and I am trying to develop some ideas about the Primordial Abyss at the moment. I have never paid any attention to 4th ed. stuff, except possibly if it doesn't conflict with classic stuff.
So if the latter is the case, what do you think His role was during the Law/Chaos War, and what was his relationship with the Obyriths and the Abyss in general for that matter?
I am toying with the idea that Tharizdun was a major enemy of Baator long before the fall of Ahriman and the rise of the Baatezu. This conflict weakened the Ancient Baatorians so severely that they were driven into hiding deep in the pits of Baator - never really recovering enough to become a major interplanar power again, and subsequently playing a greatly diminished role in the Law/Chaos War.
My take is that the Protogenoi are just from the Greek pantheon, but I guess Greyhawk also had equivalent powers and Tharizdun was one of them. His corruption probably happened hundreds of millions years ago, times when aboleths, elder things first appeared, after the first contact with the Far Realm.
Or alternatively, maybe his relationship with the Abyss comes from being interested in the draeden around which Abyss grew around, draedens despise all elemental material.