On abilities that depend on the value of damage caused, do we account for the target's resistances?

Question

The Death Kiss, in Volo's Guide to Monsters, has an ability that says

On a failed save, the target takes 4d10 (22) lightning damage, and the Death Kiss regains half as many hit points.

Yesterday, the Death Kiss used this on a Barbarian, who resisted the damage, an only took 11 points of lightning damage. I ruled that the Death Kiss still healed for 11 HP, but the players mostly disagreed, and claimed it should have healed for half the damage it caused, so only 5 HP (rounded down).

On abilities like this, do you account for damage modifiers belonging to the target or just the original value?

Answer 1

You were right - and not just because you are the DM

The words speak for themselves:

On a failed save, the target takes 22 lightning damage, and the Death Kiss regains half as many hit points.

That last part always means 11hp. The recovery of the hit points is not predicated on the target taking damage or how much damage they take. Even targeting a creature immune to lightning heals 11hp.

In this particular case, the healing does not depend on the damage caused - other abilities do depend on damage caused.

Answer 2

We consider the resistances

Death Kiss:

On a failed save, the target takes 22 (4d10) lightning damage, and the Death Kiss regains half as many hit points.

As quoted many times here from Jeremy Crawford, "if a feature was meant to work that way, it would say so."

If the creators wanted the Death Kiss to always heal 11 hit points, the description would look like this:

On a failed save, the target takes 22 (4d10) lightning damage, and the Death Kiss regains 11 hit points.

As it is now, the Death Kiss heals half as many hit points as the target takes, so 5 for a raging Bear Barbarian, 0 for a Blue Dragon.

Answer 3

Looking at vampiric Touch, a similar spell.

"You regain hit points equal to half the necrotic damage you dealt"...

And looking at the wording of this abilities I would rule this is similar to Vampiric Touch and have the hp regeneration be based on the amount of damage dealt, not o. The roll to see how much savage is dealt.

I think that ability could have more explicit wording as it can be interpreted two ways unlike vampiric torches wording.