How would making undead vulnerable to radiant damage affect balance?

Question

I really like the flavor of undead being vulnerable to radiant damage, but that seems to not be the case for many monsters in 5e. What would the practical effect be of changing that?

Obviously "they would take double damage from radiant effects", but I'm looking for how it would affect the flow of the game - what common, relevant spells or effects beyond Divine Smite and Sacred Flame do PC's have access to? Would it make paladins and clerics stronger against undead to the point that other party members would feel useless? Would a party with no clerics or paladins be affected in any meaningful way?

Obviously, I can speculate on my own, so please cite relevant experience in your answer.

Answer 1

From What are the most common damage types per spell level?, there are 14 spells that deal radiant damage, making it the 5th most common spell damage type. These are predominately cleric and paladin spells. There is at least one for every spell level except 7th and 9th so they are available early and often. Radiant damage is a good choice because few creatures are resistant or immune to this type of damage unlike, say, fire which is both the most common spell damage type and the most resisted.

That said, your question is highly dependent on how you use undead in your game. If the players never fight them then obviously it will have no effect whatsoever. If the campaign is a zombie apocalypse and that's all they fight then it will have a much greater effect. How much you telegraph what they will be facing will also be a big factor - clerics and paladins with foreknowledge can optimise their spell selection. If this telegraphing extends to pre-character creation then players may be drawn to paladin and light-domain clerics, this is not necessarily a bad thing.

As to the effect of vulnerability on CR the DMG has this to say (p. 277):

Vulnerabilities don't significantly affect a monster's challenge rating, unless a monster has vulnerabilities to multiple damage types that are prevalent, especially bludgeoning, piercing, and slashing.

This makes sense, if 1 (or 2) PCs in a party of 4 do double damage on a successful hit the effect is as if there was 1 (or 2) more PCs making attacks but there are still only 4 copping damage. The creatures will be down 1/4 to 1/2 as quick but most creatures hand out the bulk of their damage in the first couple of rounds anyway.

Of course, you are the DM, if you slip 1 more zombie into the pack or give the lich 25% more hp the effect of radiant vulnerability will be effectively cancelled out but the PC who picked paladin still gets to feel clever.

Answer 2

There are two types of game balance: PCs vs. NPCs and PCs compared to each other.

Undead radiant vulnerability is unlikely to hinder PCs in any way, but it will hinder their foes - how much depends on how many of the foes are undead and how much radiant damage the party can put out. This makes the game “easier”, but you can easily compensate by using more foes, stronger foes, or a greater variety of foes. The latter can lead to some fun tactical gameplay by encouraging particular PCs to go after particular foes in an encounter.

Balance between PCs is about making sure everyone at the table can contribute and feel powerful. Radiant damage is easy to deal for Paladins and Clerics, and should make those players feel particularly cool. Other players may feel left out; this is the real risk with this change. This can be mitigated in a number of ways - just make sure to give each player something to make them feel as powerful every now and then.