Would changing the damage types of monsters to fit a theme change the CR? For example, making a Shadow dragon deal radiant damage and transform killed people into fey work as a dragon from the feywild or a flameskull that deals psychic damage because it's the remains of a powerful mystic.
If you design monsters specifically to get around the PCs resistances (or to trick PCs who are trying to prepare), you should bear that in mind as a situational adjustment to difficulty, and in general converting damage from poison to a different type will make it more effective, but other than that no.
As others have said: Generally, no, unless you are specifically working around resistances you know your PCs have.
Just to add that some damage types are typically harder to resist. For example, PCs are less likely to find a way of resisting psychic damage than fire damage. And there is barely anything to resist force damage. So it does depend on your situation. If the PCs have access to the Protection from Energy spell, then changing a monster from breathing fire to causing necrotic damage could make the encounter harder.
In my experience damage types kind of fall into three groups.
There's the common damage types... slashing, piercing, bludgeoning, and you can swap those around as you please without really affecting things.
A step up from that are the common elemental damage types... fire, cold, lightning, thunder, and I'd include poison in that as well. These are damage types that it's common to encounter in the world and that are sometimes resisted. You can swap any one with another damage type and it's not much of a difference.
And then there are the rare damage types... not necessarily any more deadly than the other damage types, but more rarely resisted. Radiant, necrotic, psychic, and force. If I remember correctly, force is the least-resisted damage type, both among monster tables and among abilities granted to players.
I might have missed a damage type somewhere, but that basically covers it. Let's call them Tier 1, Tier 2, and Tier 3. You can mix and match Tier 2 with pretty much any other damage type and it shouldn't really change the CR of an encounter. But replacing a Tier 1 damage type with a Tier 3 damage might shift it a bit.
And then there are the rare damage types... not necessarily any more deadly than the other damage types, but more rarely resisted. Radiant, necrotic, psychic, and force. If I remember correctly, force is the least-resisted damage type, both among monster tables and among abilities granted to players.
I did a check on monsters a bit back, though it's somewhat different for players. Counting total of resistances, immunities, and vulnerabilities in the MM:
Magic Weapons: 0 immune, 1 resistant (demilich)
Force: 1 immune (helmed horror), 0 resistant
Radiant: 0 immune, 4 resistant (various celestials), 2 vulnerable, several other monsters suffer special effects.
Psychic: 11 immune (constructs of various types), 1 resistant, 1 vulnerable.
Would changing the damage types of monsters to fit a theme change the CR? For example, making a Shadow dragon deal radiant damage and transform killed people into fey work as a dragon from the feywild or a flameskull that deals psychic damage because it's the remains of a powerful mystic.
Typically it shouldn't make much of a difference, unless you're specifically targeting damage types that players do or don't have protection from.
I guess damage types that don't get reduced by a Barbarian rage would make the monster tougher. But usually not enough to change the CR, IMO.
If you design monsters specifically to get around the PCs resistances (or to trick PCs who are trying to prepare), you should bear that in mind as a situational adjustment to difficulty, and in general converting damage from poison to a different type will make it more effective, but other than that no.
As others have said: Generally, no, unless you are specifically working around resistances you know your PCs have.
Just to add that some damage types are typically harder to resist. For example, PCs are less likely to find a way of resisting psychic damage than fire damage. And there is barely anything to resist force damage. So it does depend on your situation. If the PCs have access to the Protection from Energy spell, then changing a monster from breathing fire to causing necrotic damage could make the encounter harder.
In my experience damage types kind of fall into three groups.
There's the common damage types... slashing, piercing, bludgeoning, and you can swap those around as you please without really affecting things.
A step up from that are the common elemental damage types... fire, cold, lightning, thunder, and I'd include poison in that as well. These are damage types that it's common to encounter in the world and that are sometimes resisted. You can swap any one with another damage type and it's not much of a difference.
And then there are the rare damage types... not necessarily any more deadly than the other damage types, but more rarely resisted. Radiant, necrotic, psychic, and force. If I remember correctly, force is the least-resisted damage type, both among monster tables and among abilities granted to players.
I might have missed a damage type somewhere, but that basically covers it. Let's call them Tier 1, Tier 2, and Tier 3. You can mix and match Tier 2 with pretty much any other damage type and it shouldn't really change the CR of an encounter. But replacing a Tier 1 damage type with a Tier 3 damage might shift it a bit.
Watch Crits for Breakfast, an adults-only RP-Heavy Roll20 Livestream at twitch.tv/afterdisbooty
And now you too can play with the amazing art and assets we use in Roll20 for our campaign at Hazel's Emporium
I did a check on monsters a bit back, though it's somewhat different for players. Counting total of resistances, immunities, and vulnerabilities in the MM: