Are there any attacks or effects that reduce max HP to 0 without stating what happens?

Question

Inspired by PJRZ's comment on this answer to this question:

What attacks actually exist that reduce someone's max HP, but do not specifically state what happens at max HP of 0?

As far as I am aware, all monster abilities that can reduce max HP to 0 state that the creature dies (and optionally may be brought back as some undead creature, depending on the exact creature/attack used to reduce the max HP to 0).

As an example of an attack that does state what happens, consider a Wraith's Life Drain attack (MM, pg. 302):

Life Drain. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 21 (4d8 + 3) necrotic damage. The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Are there any attacks or effect that reduce a creature's max HP to 0, but do not address whether or not the target dies?

Answer 1

There was... (prior to the 2018 errata)

Jeremy Crawford's 2017 tweet

This question regarding "death by leveling" was inspired by Jeremy Crawford's tweet that you could take a hit to your Max HP when leveling up.

The fact that you're unlikely to take such a hit unless your constitution score is very low means you're likely to have a low Max HP to start with, and having it drop to 0 when leveling up becomes a real possibility if you roll for it.

For Example:

Jeremiah, a Sorcerer, rolled so many 3s at character creation that he decided to put one into Constitution. At level one, that's 6 + (-4) for a Max HP of 2.

If he doesn't roll for Max HP gains when he levels up, 4 - 4 will never give any net gains. If he chooses to gamble for an extra point or two, however, he could drop to 0 at the very first level!

This is the most extreme example of this effect I'm aware of RAW, but the same basic effect is possible with a constitution score of 4 or 5 too. In fact, any Constitution score below 8 could leave you with 0 Max HP with enough bad rolls in a row when leveling up. Start with 6-2=4 Max, roll 4 1s, there you are.

The question you linked was asked because this didn't seem to be directly addressed in the rules. However...

2018 Errata

In 2018 the Wizards of the Coast official Player's Handbook Errata adjusted the wording regarding leveling up on p.15 of the PHB to read:

Each time you gain a level, you gain 1 additional Hit Die. Roll that Hit Die, add your Constitution modifier to the roll, and add the total (minimum of 1) to your hit point maximum. - PHB p.15 with errata

I'd consider that where there are two official rulings that definitely contradict one another, the later ruling would supersede the earlier one. This means that Jeremiah can no longer lose hitpoints when he levels up.

An interesting follow on effect: It seems to me that the text regarding taking the fixed average (instead of rolling your hit die) that completes that paragraph does not change the "minimum of 1" that is applied to the total:

Alternatively, you can use the fixed value shown in your class entry, which is the average result of the die roll (rounded up) - PHB p.15

So there's not even that way to get a zero gain, but rolling, for Jeremiah, which used to carry the risk of death, now cannot be worse than taking the average - rolling: 1d6 + (-4) (minimum of 1) yields 1 or 2, but the average: 4 + (-4) (minimum of 1) always yields 1. Not a big difference, but it still means that what used to be a risky move becomes the obvious best and safest!

Answer 2

Yes (indirectly): clay golem + aid

The clay golem's slam attack (MM p. 168) has the following effect:

If the target is a creature, it must succeed on a DC 15 Constitution saving throw or have its hit point maximum reduced by an amount equal to the damage taken. The target dies if this attack reduces its hit point maximum to 0. The reduction lasts until removed by the greater restoration spell or other magic.

On its own, reducing a creature's hit point maximum to 0 with this attack would cause a creature to die. However, note that, unlike other effects, the condition is 'if the attack reduces...to 0', rather than the much more open-ended 'if this effect reduces...to 0'. We can therefore use the aid spell (PHB p. 211) to circumvent this condition:

Each target's hit point maximum and current hit points increase by 5 for the duration.

If a creature has the aid spell cast on it, then is hit by (and fails the save associated with) the slam of a clay golem, having its hit point maximum reduced to 5, and then the aid spell expires, the creature's hit point maximum—and current hit points—would be reduced to exactly 0 without triggering any condition that would instantly kill it. Additionally, since the creature didn't take damage when it dropped to 0 hit points, it is also unclear whether the creature falls unconscious, dies, or neither.