Vulnerability, Immunity and Dragon Mask

Question

In our D&D 5e game, we are lvl 20 characters with a lot of magical item and something weird happened.

To start, I learned that resistances and vulnerability doesn’t actually cancel each other. You in fact consider both in this given order: Resistance first, followed by Vulnerability, making it possible that you actually take 1 less damage than if they were to cancel out. (If the initial damage is odd)

My question is: I now have vulnerability to fire damage due to the « Tree » card from the deck of many more things. I also got immunity to fire damage from the « Boon of the fire soul ». Turns out I also have the « Red Dragon Mask ». Due to all those things is there a weird interaction with all those things where I would get healed by the full initial amount instead of half of it?. Example: I get hit for 10 fire damage. Vulnerability doubles it to 20. Immunity + Fire Dragon Mask transform it into 10 healing.

Or on the other side: I get hit for 10 fire damage. Immunity + Fire Dragon Mask transform it into 5 healing. Vulnerability doubles it to 10.

Answer 1

dnd-5e

You are healed for half the damage

In the PHB section on Damage Resistance and Vulnerability, we are told:

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

If vulnerability is applied after all other modifiers to damage, and if we consider Immunity to be another modifier to damage, then we know that we must apply Immunity before vulnerability.

Your Boon of the Fire Soul (DMG 232) gives you immunity to fire damage.

Your Red Dragon Mask says:

If you already have immunity to that damage type from another source, whenever you are subjected to damage of that type, you take none of that damage and regain a number of hit points equal to half the damage dealt of that type.

Being subjected to fire damage, you are Immune from your Boon, which triggers the healing from the Mask at half the damage you were subjected to - and all of these effects take place before your vulnerability.

By the time your vulnerability comes into play, your damage has been reduced to zero, so your vulnerability itself never triggers.