I'm worried about nerfing the Players too much by adding an immunity to Radiant Damage to a monster type

Question

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.

Answer 1

It would be a nerf, and is not needed for your narrative

Game Mechanics

I think that giving demons outright immunity to radiant damage is both slightly unbalanced considering there is a number of common spells and effects especially from divine character classes that deal radiant damage (such as sacred flame or spirit guardians) and it also does not fit the design sensibility of 5e.

Looking at the 36 demons that have been published in official sources (MM, Mordenkainen's Tome of Foes, Mordenkainen Presents Monsters of the Multiverse, Volo's Guide to Monsters), by far the most common resistance / immunity pattern is resistance to cold, fire, lightning, non-magical weapons and immunity to poison, shared by 11.

Other than immunity to poison, energy damage type immunities are exceedingly rare. The Shadow Demon has cold and lightning, the Balor has fire immunity, the demon lord Juiblex has acid and mighty Orcus himself is immune to necrotic damage, but that's it, as far as I can tell.

So the only immunity you could remove to "balance" this is poison, but poison is the damage type with by far the most resistant and immune creatures, as many poisonous beasts and other creatures, as well as undead, constructs and other outsiders like devils are also immune to it. That, plus the fact that basic poison is ridiculously overpriced for it meager damage effect means that it is very unlikely to have player characters that focus on poison damage. So swapping them, you can expect to weaken the party's ability to damage the demons by granting immunity.

If you wanted to make the demons better able to tolerate radiant damage, you could give them resistance to it instead. It still is somewhat of a nerf, so maybe drop one of the other less common resistances, like lightning. (I would not drop fire or cold: as there are a large number of spells dealing those damage types, doing so would likely weaken the demons relatively).

Narratively

The spear of light is unlikely to fill the entire abyss with radiant damage: it will pierce the layers at a given location, and there are mountains, chasms, caves, buildings and all kind of weird growth to take shelter behind or under from light that would emanate from it. Also, if the light was severe enough to deal radiant damage, that would be lethal for any form of life, beasts, plants, PCs included. So unless all of the Abyss is now a rather barren wasteland, demons would have no need to be even resistant to such damage.

Moreover, as molot points out, if the demons were immune to radiant damage, they would not suffer from it at all, so the whole idea of them having to endure that light would not work.

Lastly, even while D&D is not a physics or biology simulation, if you look at the lifespans of demons (do they even die?) and the time scales in which evolution is able to adapt a species to changed environments (many, many generations), there is no way that a few decades of searing light would be enough to allow the demon population to develop any kind of resistance or immunity. (Although granted, demons' forms are of chaos, and could be quite mutable.)

My recommendation both on narrative and on game mechanics grounds is to just let the demons be like they are in the rules. If they get to close to the spear, they will suffer. So they will try to avoid doing that.

Answer 2

It's your world - it's not a nerf

If this is the story you want to tell, then go ahead and tell it! Making that change to damage resistances/vulnerabilities is 100% part of the DM toolkit and something you can do. Tailoring the monsters to fit your story is not only allowed, it's encouraged!

Changing the stats may alter the CR, so just be aware when designing your encounters as they may be more difficult than originally planned with the 'base' monster.

DM's don't get a lot, but what we do get is the ability to control the overall world and what monsters and encounters the players face. If we remove the ability to alter monsters to tell our story because of a fear of nerfing, then we're vastly limiting the tools available to a DM to have fun and build not just encounters, but a shared and new story with the players.

Why not consider this a nerf?

If you're worried about 'nerfing', then I'd start with asking "Why?"

Do you have a player with a character that specializes in radiant? Is the entire campaign going to be somewhere where a major class feature just doesn't work?

If the answers the above-two are 'no', then go for it! It's not a nerf, it's just those particular monsters and how you deal with them. If the answer is yes (to either of them), then you're nerfing the players.

Creating a customized monster is not a nerf. Creating a customized monster to counter a specific character is a nerf (unless there's a story-reason for it.)

Personal experience with surprise radiant immunity

I've played a vengeance paladin and come against monsters that were immune to radiant. It was a surprise, but that's part of the game. I'm not always going to get to do my best, and the spotlight can and should move around the party.

Communication

If you do make these changes, and the characters know of the 'previous' form, and not these, then make sure to seed bits of information around and consistently so it's not a surprise. If it is a surprise because they don't know what's going on, then that's okay, too - but make sure the first combat you provide some clues that something is different. Subverting expectations (or players who know the monsters already), is okay, but giving context clues makes it much more enjoyable for everyone.

Answer 3

This is a nerf. There's no way around that.

However, one of the ways I've worked around this in the past is by having a way for players to work out how to remove that immunity in-battle.

Think along the lines of a magic totem that's granting the immunity that, when destroyed, the immunity goes or there's a bunch of weaker minions casting spells that are granting the immunity that, when killed, the immunity goes.

If you can weave this into your narrative, even better. I'd also try to give your players a hint that this might be something to do with it (like "there's a strange glow around the enemy that seems to be emanating from a nearby totem") rather than outright telling them "oh if you destroy this, it will stop the immunity". That way they get a sense of accomplishment from working it out, even if it was fairly obvious.

This may or may not be in-line with how 5e is typically played... But you're the DM, if you think the players will enjoy the challenge, go for it!