What happens when a creature drops to 0 hit points without taking damage?

Question

The rules on dropping to 0 hit points (PHB p. 197) almost entirely depend on taking damage. The section begins as follows:

When you drop to 0 hit points, you either die outright or fall unconscious, as explained in the following sections.

However, we then have the two following sections which make reference to dropping to 0 hit points:

Instant Death

Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

Falling Unconscious

If damage reduces you to 0 hit points and fails to kill you, you fall unconscious (see the appendix). This unconsciousness ends if you regain any hit points.

In fact, the only reference which doesn't include the condition of damage pertains to monsters:

Monsters and Death

Most DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.

The only rule pertaining to player characters which doesn't involve damage is what happens after one is at 0 hit points:

Death Saving Throws

Whenever you start your turn with 0 hit points, you must make a special saving throw, called a death saving throw, to determine whether you creep closer to death or hang onto life.

All emphasis mine. All of this raises the following question: in the absence of assuming intent (however clear it might seem), are there any rules for determining what happens to a player character who drops to 0 hit points as the result of an event which does not involve damage, such as the expiration of the aid spell?

Answer 1

The creature either dies, or falls unconscious

The rules say (PHB p. 197):

When you drop to 0 hit points, you either die outright or fall unconscious

Dying outright

When does the creature die outright? In two cases: either due to massive damage, or because the creature is a monster and the DM does not want to make death saving throws for it as it is too much of a hassle that does not contribute to gameplay.

When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

DMs have a monster die the instant it drops to 0 hit points, rather than having it fall unconscious and make death saving throws.

Falling unconscious

In all other cases, the creature does not die outright. It logically follows that the creature then falls unconscious. There are only these two options, to die outright or fall unconscious.

The rules confirm this happens in case of damage reducing you to 0 hit points explicitly, and generally:

If damage reduces you to 0 hit points and fails to kill you, you fall unconscious

This only leaves the case if you are reduced to 0 hit points from sources other than damage (like the dropping of of Aid). Do you fall unconscious or die?

This case is not covered explicitly and will require the DM to make a judgment call. The treatment for damage sources suggests that unconsciousness is intended as the default effect of being at 0 hit points, as it indicates there needs to be something special about the damage to kill you. It would stand to reason that the same logic, of non-damage sources reducing your hit points likewise require something special to kill you, but it is not airtight.

Once unconscious, the creature has to start making death saving throws on subsequent turns, and then will either stabilize, on a lucky 20 awake with 1 hit point, or die.

Special cases

There are a few special effects that lead to other outcomes when you fall to 0 hit points, for example, disintegrate:

The target is disintegrated if this damage leaves it with 0 hit points

All these cases are applications of the specific beats general rule, where a more specific rule overrides the general rule of either dying of falling unconscious.

Answer 2

Depends on why they lost hit points

Per the Basic Rules, again, hit points, "represent a combination of physical and mental durability, the will to live, and luck."

So hit points can go down for any number of reasons, but mostly because of damage.

There are other methods to "go to 0 hit points" without damage.

For instance, the Night Hag:

Nightmare Haunting (1/Day). While on the Ethereal Plane, the hag magically touches a sleeping humanoid on the Material Plane. A protection from evil and good spell cast on the target prevents this contact, as does a magic circle. As long as the contact persists, the target has dreadful visions. If these visions last for at least 1 hour, the target gains no benefit from its rest, and its hit point maximum is reduced by 5 (1d10). If this effect reduces the target's hit point maximum to 0, the target dies, and if the target was evil, its soul is trapped in the hag's soul bag.

It's discussed a bit more in this question.

But there is no one-answer-fits-all as 99% of hit point reduction is from what is called "damage". The 1% will be through special rules and will need to be on a case by case basis.

Not every creature reduced to 0 maximum hit points will end up in a Night Hag bag.

But as a general rule, "When you drop to 0 hit points, you either die outright or fall unconscious, as explained in [the following sections]."

No mention of damage, so at 0 you're either unconscious or dead.

Answer 3

The rules don't say what to do

The rules for dropping to 0 hp state:

When you drop to 0 hit points, you either die outright or fall unconscious, as explained in the following sections.

It goes on to say:

When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.

If damage reduces you to 0 hit points and fails to kill you, you fall unconscious

However what happens when you are reduced to 0 hp by something other than damage is not defined by the rules.

Imagine if a recipe book said "you can make this recipe with beef or tofu; if you want to eat 500 calories then use beef, if you are vegan use tofu" - if you don't have a calorie goal in mind and you aren't vegan then the recipe doesn't offer any guidance, but the choice still remains.

The DM should make a ruling

Whenever we reach the edges of the rules, that's where DM rulings come in.

You will probably find that in some situations it is appropriate to rule that the creature dies instantly, and in others they should fall unconscious. The exact ruling will depend on the situation, although I can tell you that I have never had to make this ruling before despite playing this game for about a decade.