Edowir says that:
"demon" -> "Demons are the servants of evil. More or less devoted servers of Zathroth they cause strife and havoc wherever they appear. Their masters are known as Demonlords, Demon Overlords and Archdemons."
"demonlords" -> "Demonlords are the generals of their kind. They are more cunning than ordinary demons, and they can channel their hatred more effectively than their lesser brethren, making them even more formidable opponents."
"demon","overlords" -> "The overlords of the demonkind are more powerful than even demonlords are. They are nearly indestructible. Armoured with layers of impenetrable hide and endowed with awesome magical power, demon overlords are true incarnation of death."
"archdemons" -> "The archdemons are few, and they are extremely rare. And a good thing, too, for they are the rulers of the demonrace. They are vain and powerhungry creatures who tend to form only small cabals and fight each other instead of allying up against creation."
"cabals" -> "There are at least five demonic cabals of archdemons. The ruthless seven are the most prominent and powerful."
Making it kinda the hierarchy of
cabals (as group is stronger) > archdemons > demon overlords > demonlords > demons > lesser demons
Edowir mentions that archdemons are the triangle of terror guys, and also that the triangle of terror is in fear of ruthless seven
so the cabal of ruthless seven must be higher than the archdemons, i mean at all they are controlling the cabal of archdemons, and they have their own minions who are archdemons too
probably thats would be my guess that the ruthless seven were the demon overlords in past, and soon as they gained stength they summoned the minons (inq bosses) making ruthless seven ultimetly the archdemons.
Zathroth could have created the first demon overlord from twin prototype of banor, where the banor was created to get rid of Urgith the god of undeath.
Since all ruthless seven guys present like this:
Apocalypse, Total Destruction
Ashfalor, the right hand of Urgith. The general of the undead hordes.
Bazir, the Great Deceiver, Lord of the Lies
Infernatil, the Incendiary of Hell
Pumin, the Lord of Despair
Tafariel, the Mistress of the Damned
Verminor, the Plaguelord
My guess will be that then, if i state that ruthless seven were demon overlords in past, so the first demon overlord to be born was the Ashfalor, the right hand of Urgith to help him counter the Banor, as his twin brother.
But yet, this is just my research and my personal opinion.