[RPG] What happens when a creature with vulnerability to a damage type has resistance to the same type imposed on it, or vice versa


So, let's imagine for a moment that two Fiend-patron Warlocks are fighting. One uses its Fiendish Resilience to get resistance to fire damage, but the other one casts the Hallow spell with the Energy Vulnerability effect to impose vulnerability to fire damage.

The description of the Fiendish Resilience feature:

Fiendish Resilience: Starting at 10th level, you can choose one damage type when you finish a short or long rest. You gain resistance to that damage type until you choose a different one with this feature. Damage from magical weapons or silver weapons ignores this resistance. (PHB 109)

And the extra effect of Hallow:

Energy Vulnerability: Affected creatures in the area have vulnerability to one damage type of your choice, except for bludgeoning, piercing, or slashing. (PHB 249)

Which trumps which in this scenario? Or do they cancel out and result in standard (1x) damage?

I'm sorry if this has already been asked; I didn't see anything like it in the "Related Questions" or "Similar Questions" sections.

Also, bonus points:

  • If imposed vulnerability/resistance trumps innate vulnerability/resistance, how does it interact with immunity?

Best Answer

They Both Apply (ALMOST 1x damage)

On page 197 of the Player's Handbook, it states (bolds added):

Resistance and then vulnerability are applied after all other modifiers to damage.

This means that you would apply resistance, and then vulnerability to damage, in that order. This often would mean that you would only take the normal (x1) amount of damage: but not always. Note that on page 7 of the PHB, it states:

There’s one more general rule you need to know at the outset. Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater.

As such if the damage dealt was odd (eg, 15) first it would be halved, by resistance, rounding down (eg, to 7 from 7.5). This halved damage would then be doubled by vulnerability (eg, increasing it to 14).

DMs might ignore this fringe case, as it can only result in a 1 point damage difference. But it is an accurate feature of the rules as written.

As far as Immunity goes, Vulnerability/Resistance both apply to enemies who also have Immunity, but to no particular effect. After all, halving or doubling 0 will still result in 0.