Can a Vampire bite's HP drain deal extra damage to a creature with temp HP?

Question

Can a Vampire's bite deal extra damage due to temporary hit points absorbing the initial damage?

The Vampire's Bite attack contains the following (SRD p. 353):

Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount.

So say I have a max HP of 20, with 20 hit points and 20 temporary hit points. The Vampire uses Bite dealing 5 piercing damage and 10 necrotic damage (reducing temp HP to 5 now).
Does my max HP get reduced by 10, thereby causing me to take 25 damage instead of the expected 15?

Pre-Bite:
Max HP = 20
Current HP = 20
Temporary HP = 20

During Bite (damage only):
Max HP = 20
Current HP = 20
Temporary HP = 20 - 5 (piercing) - 10 (necrotic)

Post Bite (Max HP reduction):
Max HP = 10??
Current HP = 10??
Temporary HP = 5.

How do temporary hit points and a Vampire's Bite interact?

Answer 1

Yes, a vampire's bite can sometimes effectively deal extra damage due to temporary hit points

Damage to temporary HP still counts as damage

Per this Jeremy Crawford Tweet:

When temporary hit points absorb damage for you, you're still taking damage, just not to your real hit points.

So, when the bite ability says "hit point maximum is reduced by an amount equal to the necrotic damage taken" that means that max HP will be reduced by the amount of damage done regardless of temp HP.

Your example

In your example, your maximum HP is 20. Your current HP will also start at 20 since you have taken no damage before this. You also start with 20 temporary HP from some spell or effect.

Starting condition

Max HP: 20

Curr. HP: 20

Temp. HP: 20

So, you take 5 piercing damage and 10 necrotic damage.

Max HP: 20

Curr. HP: 20

Temp. HP: 20 - 5 (piercing) - 10 (necrotic) = 5

Then, because of the vampire bite, your maximum HP is reduced by 10.

Max HP: 10

Curr. HP: 10

Temp. HP: 5

And because your maximum HP drops below your current HP, your current HP immediately drops with it. This is because current HP can never exceed max HP:

A creature's current hit points (usually just called hit points) can be any number from the creature's hit point maximum down to 0. This number changes frequently as a creature takes damage or receives healing.

In this case then, a **vampire's bite, will do additional damage to your HP because of the HP drain and the fact that you had temporary HP.

Specifically it did:

• 5 (piercing)

• 10 (necrotic)

• 10 (HP drain)

= 25 damage

A creature without temp HP would have taken 15 damage. So, yes a creature with temp HP in this case actually effectively takes more damage than a creature without.

Temporary HP is not affected by HP drain since it is independent of max HP.

Official ruling

Jeremy Crawford has agreed with this interpretation in this tweet:

Q: You are hit by a Specter's Life Drain for 10 damage. You have 8 temporary hit points. Is your Max HP lowered by 2 or by 10?

A: By 10

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Note that I have been referring to the HP maximum reduction as "damage". I do not mean damage in the technical sense (eg effects that depend on "you take damage") but in the common sense that you lost HP thus it is effectively damage.

Answer 2

The victim's hit point maximum in your scenario is reduced by the full amount of the damage inflicted, even if the creature takes no actual HPs damage due to it being "absorbed" by temporary HPs.

As @Shem points out in the comments below, Jeremy Crawford has agreed with this interpretation in this tweet:

Q: You are hit by a Specter's Life Drain for 10 damage. You have 8 temporary hit points. Is your Max HP lowered by 2 or by 10?

A: By 10

So adopting the interpretation detailed below (my original answer prior to seeing the official ruling from JCrawford) would be a house rule. I leave it here to show the history of answering this question and for possible minor interest...

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A House Rule alternative to the official ruling: The victim's hit point maximum in your scenario is unaffected.

Vampire Bite (Monster Manual p.297)

Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount.

If a target has temporary hit points then any damage taken is first deducted from these and the target is protected from injury. Temporary hit points are not real hit points and are lost first, with real hit point damage only taken if there is leftover damage.

Temporary hit points (PHB p.198)

Temporary hit points aren’t actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.

When you have temporary hit points and take damage, the temporary hit points are lost first, and any leftover damage carries over to your normal hit points.

So your scenario:

So say we have Max HP 20, 20 Hit Points, and 20 Temporary Hit Points, and the Vampire uses Bite dealing 5 Piercing Damage (THP = 15 now) and 10 Necrotic Damage (THP = 5 now).

This leads to the following outcome:

• The vampire bites for 5 piercing and 10 necrotic, all absorbed by the temporary hit points, leaving 5 temporary hit points and the character uninjured.
• No necrotic damage was done to the character as it was all absorbed by the temporary hit points (no damage was done at all, the character was protected from injury) so their maximum hit points are unaffected.
• The victim's hit points remain at 20
• The victim's maximum hit points remains at 20

However what about other scenarios? Let's say the attack happened again next round with exactly the same damage, except now the character only has 5 temporary hit points:

• The vampire bites for 5 piercing and 10 necrotic damage, the victim has 5 temp hit points
• The DM rules that the piercing damage is done "first" with the necrotic done second as the blood/life is sucked from the victim.
• The victim's temp hit points absorb the 5 points of piercing damage completely leaving no temporary hit points.
• The victim takes 10 hit points of necrotic damage
• The victim has 10 hit points remaining (20 -10 necrotic)
• The victim's maximum hit points is reduced by 10 (the necrotic damage done) to 10

A different DM ruling might be that the temp hit points are "shared out" in the ratio of the different damage types done as it all happens at the same time. This takes more maths so is slower and cumbersome, but in some circumstances makes much more sense (a Flame Strike where the fire and radiant damage is done at the same time and there is no story telling "order" or fluff saying otherwise), though in my opinion for the vampire's bite scenario the ruling above makes more sense, both thematically and in terms of smoother gameplay.

However if the DM went with the "shared out" ruling:

• The vampire bites for 5 piercing and 10 necrotic.
• 5/15 or 1/3 of the temp hit points absorb piercing damage and 10/15 or 2/3 absorb necrotic damage
• The DM therefore rules that the victim takes 3 piercing and 7 necrotic damage
• The victim has 10 hit points remaining (20 -3 piercing -7 necrotic)
• The victim's maximum hit points is reduced by 7 (the necrotic damage done) to 13

On a final note, this character, with 20 hit points and 20 maximum hit points and 20 temporary hit points stands no chance against a vampire and I feel very sorry for them.