Demons don’t want souls. At least not how you’d understand it in a conventional sense.
There are souls that go down to their realm, yes. And many of those lost ghosts are comforted by those that understand what it means to be exiled from the sublime realms. The souls transform into an orchard of spirits, left to time and company to heal from the outrages of the material world.
Because, you see, a soul is not experienced by demons as a singular thing. It is a completed line across space-time.
Those who lead the most vile of lives narrow their options within the scope of their existence. They spearhead their destination to an inevitable point.
But those who SOLD their souls. Those brought their lives to a razor-sharp end.
You see, most souls rejected by Heaven are protected by the Fallen, because they know what it is like to be thrown in the muck. But those last two categories… the vile and the damned.
Those souls are WEAPONS.
The war against a cruel and impartial Creator never ended. The Fallen will rise with the very tools that the Creator Himself rejected.
And this time they will win.