Fading Soul...

Pictured is the remnants of a demon's soul erased by angelic weapons, roaming the anger layer until it fades away.
When a demon is killed by an angel's holy power (or weapon infused with such) they return to their layer of hell as usual,
but fail to reform their physical body from the ground, leaving their soul to wander the plane for days, losing their selves piece by piece until they fall,
fading away forever.

The souls of demons are greatly different than human's souls. Vastly weak, unless they make a identity differing from the masses.