Interaction clarification with 'Invoke curse' and temporary hitpoints

Question

I have recently gotten to the end of a Tomb of Annihilation campaign as a player (potential spoilers ahead) and during our final fight, a magic item was used to apply the following effect to our party:

Invoke Curse. The Staff of the Forgotten One has 7 charges and regains 1d4 + 3 expended charges daily at dawn. While holding the staff, you can use an action to expend 1 charge and target one creature you can see within 60 feet of you. The target must succeed on a Constitution saving throw (using your spell save DC) or be cursed. While cursed in this way, the target can't regain hit points and has vulnerability to necrotic damage. A Greater Restoration, Remove Curse, or similar spell ends the curse on the target.

During this fight, our party also had a different affect that gave temporary hit points to some of our members each round and at the time I argued (unsuccessfully) that temporary hit points are different from hit points as they function differently and are not a substitute for actual hitpoints. My DM said that ultimately their intention was to have this curse deny the temp hp so that we as players had to try to plan around it as a hazard.

We did ultimately beat the final fight and come out alive and I fully respect our DMs decision about denying new temporary hit points given that they'd balanced the fight around it, but from a "rules as written" perspective, which of us had the correct reading/understanding?

Answer 1

You can never regain temporary hit points

The rules for temporary hit points (p. 198, PH) say:

Healing can’t restore temporary hit points, and they can’t be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones.

That is, temporary hit points are never regained. They are lost, and new ones are gained, or they are replaced by new ones.

It does not really matter for Invoke Curse if temporary hit points are hit points, or not.¹ What matters is that you never regain temporary hit points, and all that Invoke Curse prevents is regaining hit points. So Invoke Curse does not stop getting temporary hits.

---

¹ I think one could argue either way if temporary hit points are hit points, or not. They are not real hit points according to the rules, but does that mean they are not hit points at all? They are still called "hit points" after all.

If you have a fake knife, say a theater one with a retracting blade mechanism on a sharp blade that still can cut, is it still a knife? Much like that knife, temporary hit points work like normal hit points in some ways, for example damage reduces them. And they work different in other ways, for example, you cannot heal or restore them with cure wounds and they do not stop you from being unconscious.

If you considered them to still be a kind of hit points, and Invoke Curse said it stopped you from gaining hit points, then it could prevent you from getting temporary hits. But it does not say that, so we do not need to make that assumption.

Answer 2

Temporary hit points aren’t hit points

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

In addition, they are never “regained” - you get them and you lose them, you can’t get them back.

Healing can't restore temporary hit points, and they can't be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones.

For both these reasons, the staff has no effect on temporary hit points.