The following is a variant rule to overhaul damage resistance and vulnerability for the Reborn World campaign setting for Dungeons and Dragons 5th Edition. This work-in-progress playtest content is compatible with current 5e rules.

Using Resistance Levels for damage calculation

A creature’s Resistance Level for a specific type of damage will influence how damage that creature takes of a specific type is amplified or reduced.

The level represents how many times damage should either be cut in half for Resistance or doubled for Vulnerability. If a creature gains Resistance to a damage type, increase their resistance level by +1. If they lose that Resistance or gain Vulnerability, reduce their resistance level by -1.

A single level of Resistance to a damage type will reduce the damage taken to one half the original value. Multiple levels of Resistance will stack, so a creature with two levels of Resistance to Fire damage will only take one fourth the original value of any fire damage dealt.

Levels of Vulnerability work the same way, only amplifying damage rather than reducing it. One level of Vulnerability will double damage taken of a particular type, and multiple levels of Vulnerability will also stack.

One level of Resistance and one level of Vulnerability will cancel out. If a creature has one level of Resistance to Cold damage and an effect also gives it a level of Vulnerability to Cold damage, the Vulnerability will double the halved damage and the creature will take Cold damage normally.

Immunity

Immunity is a special tier of Resistance Level. If a creature is Immune to a certain damage type, that type of damage is multiplied by zero and the creature will take no damage of that type regardless of what other levels of Resistance of Vulnerability are present.

Author’s Notes

As far as I am aware, the base rules don’t spell out any way for resistances or vulnerabilities to stack. Something is simply resistant, vulnerable, or immune with no interaction or tiers in between. This could just be because I play too much Pokemon, but I’ve always felt it would make more sense to allow Resistances to stack. Namely with player characters.

Players want to do crazy things like stack multipliers and push the system to its limits, so I believe the system should give players ways to stack up multiple layers of resistance to certain damage types. It makes complete sense for a fire character to keep resisting fire further and further to the point where it can’t harm them anymore.

This is another one where I figure players and DM’s probably run like this anyway. But I feel it’s always good to spell the option out to benefit people who don’t know about this limitation.

Feedback and playtest data are always appreciated!