[RPG] Should “resistance to damage from non-magical weapons” continue to affect monster CR once the party has magical weapons


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?

Best Answer

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."