In my setting most of the Outer Planes have individually suffered apocalyptic catastrophes.
The Abyss has been, for several decades before the players even see their first demon, beset by chronic injury that jeopardizes the stability of every ecosystem therein.
A river of holy light in the shape of a spear as long as eternity, pierces the Abyss, Vlad Tepes style, ripping a hole through every layer. Always glowing, and never ceasing. Whatever made this is long gone, as is the knowledge to undo it, so the Abyss endures, and its chaotic inhabitants manage to survive extinction.
With this kind of backdrop, I'd imagine any demons that continue to make a living in the Abyss would need to adapt to the drastic change in their ecosystem. To represent this I considered giving Immunity to Radiant Damage to any monster that hails from this plane. However I fear that because some classes and builds depend on how few creatures from the MM resist Radiant Damage in their balance, I may make some players feel ineffective.
I do want to point out Demons may be the occasional encounter for the party, but my current plans for the overall arc for this cast of characters doesn't even need to travel to the abyss. Also this is the only one of the lower planes I planned on such a side effect. Hence any devils thinking "now's our chance" would have to deal with the radiation without any resistances added. So fun time Abyss invasions are as difficult for the time being.
The current campaign may not even take the players to the Abyss, but any survivors from there will need to have mitigated a constant bombardment of hostile energy now prevalent in their home. So I want that to be reflected in any Demons I run. (Except any Demon Lords)
Would it be balanced to remove a different resistance or immunity if I add the radiant one in its place?
Would it be fine to just give them radiant resistance instead, and they muscle through the pain that is their home plane?
Or should I just apply the immunity to all the non Lord demons and my players will have to adjust any preconceived tactics as they progress?
Additional advice on any of these approaches, or even other suggestions would be greatly appreciated.