Should "resistance to damage from non-magical weapons" continue to affect monster CR once the party has magical weapons?

Question

My 5e group has reached level 5 and all have acquired +1 weapons. This renders the monster ability of "resistant to bludgeoning, piercing and slashing damage from non-magical weapons" a moot point. It's possible they'll wind up in circumstances where they need to use improvised weapons, but for the most part, this monster ability isn't going to affect play.

This is unlike resistance to Fire or Lightning, in that those resistances don't go away when the players acquire gear.

The Effective Hit Points Based on Resistances and Immunities table (DMG 277) provides a multiplier based on monster CR, but doesn't address party level, which is a pretty strong indicator of the likelihood of the party having magic weapons.

Should I continue to use these now-moot resistances when calculating effective monster HP for purposes of determining CR?

Answer 1

Do you reduce the CR of monsters with resistance to Fire or Lightning when facing a party with no Fire- or Lightning-based attacks, since the resistance doesn't affect that party?

I'm going to guess that, no, you don't, even though it doesn't affect play. Non-magical weapon resistance is no different. Some parties will be affected by the resistance. Other parties will not. But the monster remains the same, and should have the same CR, either way.

Madness lies down the path of adjusting CR to discount abilities which don't affect the PC party: "We need to have one spell of every element, so that all elemental resistances will contribute to CR. We also need one person with a non-magical weapon, so that non-magical weapon resistance will contribute. Etc."

Answer 2

The scenario you describe is a perfect example of the Tiers of Play discussion on page 15 of the player handbook. This edition of D&D is divided roughly into four tiers, the first tier comprising the first four levels, and the second tier comprising levels 5-10.

Upon reaching 5th level, characters gain access to 3rd-level spells, find valuable magic items, and pick up class abilities that completely change their effectiveness and style of play. Foes that were once quite challenging due to abilities such as resistance to non-magical weapons, are now not so threatening.

The distinction here is that the creatures are no less threatening to the community at large, but are far less threatening to your players simply because your characters are on a whole new tier of play.

But does that make the foe less worthy of granting XP?

Indeed it does; however, this has already been accounted for in the Character Advancement table (also on page 15). Advancing from 5th-level to 6th-level requires an additional 7,500 XP, which is more XP than your characters accumulated to progress all the way from level 1 to level 5. It takes a lot more of this now-inferior foe to contribute meaningfully to your characters' progress. As such, there is no need to manipulate its CR.

Answer 3

I would say yes, the CR should stay the same. Why? Let's say you had all encounters for the entire campaign pre planned, pre written, and already established. You have creatures spread out through your campaign with said resistance to non magical Blunt, Piercing, and Slashing damage. Just because your players have found magical weapons and can now pass said resistance, doesn't take that away from the creature. it's an inherent ability, it exists, and thus it counts... be it moot or otherwise. What happens if they break their +1 weapons in some other encounter? Or they use some other means of launching an attack without using their magical weapons, then it still helps to know that piercing damage from that arrow trap they pushed the monster into deals 1/2 damage, not full damage, to the creature with said resistance.

The charts/tables are designed that way for a reason. They can't possibly take all party levels and items found into consideration...it's too much randomness. Characters in a campaign I'm playing in are level 5 nearly 6 and NO magical weapons have been found so far. So those resistances would still be effective against our group. Meanwhile in the Campaign I'm running; I've designed one magical item targeted at each party member for them to find at various stages in the game.

The other thing is, nothing is stopping you from doing a test run and seeing how it goes. If you think the encounter begins to swing largely in the players favor because they're bypassing the resistance at level 5? Feel free to change the CR rating and/or give it more HP and things of that sort to toughen up the encounter and make up for the sudden lack in defense it used to have.

Answer 4

I'm in a situation that is similar, and my answer has been to simply add the multiplier to the health of the enemies and remove the resistance to non-magical weapons.