What happens if the initial damage from the Feeblemind spell reduces the target to 0 HP (or kills them)?

Question

The description of the feeblemind spell says:

You blast the mind of a creature that you can see within range, attempting to shatter its intellect and personality. The target takes 4d6 psychic damage and must make an Intelligence saving throw.

On a failed save, the creature's Intelligence and Charisma scores become 1. The creature can't cast spells, activate magic items, understand language, or communicate in any intelligible way. [...]

Does the target have to make the saving throw if they are reduced to 0 HP by the damage? Or is it failed automatically?

All creatures make death saving throws in our campaign.

Answer 1

Apply both effects simultaneously

Feeblemind says (emphasis mine):

The target takes 4d6 psychic damage and must make an Intelligence saving throw.

The "and" tells you that both effects happen to the target. You can apply them in either order, because barring some other effect, one will not affect the resolution of the other. If you were meant to apply one effect after the other, the spell description would include 'and then' to indicate which effect came later (see, for example, the importance of 'then' in the answers to Does Chain Lightning deal damage to its original target first?).

Consider the spell description of lightning lure (emphasis mine):

The target must succeed on a Strength saving throw or be pulled up to 10 feet in a straight line toward you and then take 1d8 lightning damage if it is within 5 feet of you.

Here it is clear that a failed Str save first pulls the target toward you and then, after the pull is resolved, does damage (if the target is close enough). Had the spell simply said, like feeblemind, that the target is pulled 'and' takes damage then both effects would occur at the same time and could be resolved in either order.

As you point out, the damaging effect could make the target unconscious. However, unconscious creatures receive unmodified Intelligence saves, so the order of resolution would still not matter¹.

From Conditions (Unconscious):

The creature automatically fails Strength and Dexterity saving throws.

As another example of a spell that does apply first one effect, then another, we have disintegrate. While it does not use the word 'then', it clearly indicates that the spell has an additional effect only if the results of the first, damaging effect leave the target with 0hp:

A creature targeted by this spell must make a Dexterity saving throw. On a failed save, the target takes 10d6 + 40 force damage. The target is disintegrated if this damage leaves it with 0 hit points.

Since Feeblemind neither includes 'then', nor specifies that anything special happens at 0hp, we can conclude that the save for the feebleminding effect is made independent of, and regardless of the results of, the damage.

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¹_{The order of application might matter if the damage killed, rather than rendered unconscious, the target. This might happen if the target was an NPC that died at 0hp, if the spell was cast on a creature already making death saves so that the damage caused a fatal fail, or for a creature whose maximum hp were so low that the damage caused instant death. In this case death should not protect the creature from what is supposed to be a simultaneous effect, so the save should be required before the damage is done. Further, note that spells which restore a creature to life specify which conditions and effects they remove, so it is reasonable to assume that the target would still be under the effects of the feeblemind even after its death and later revivification, raising, or resurrection.}

Answer 2

Simultaneous Effects

Feeblemind says

The target takes 4d6 psychic damage and must make an Intelligence saving throw.

Not "And then" but "And." Both of these effects happen at the same time.

Now, that said, most DMs do resolve effects sequentially, just for the purpose of managing dice rolls... So, one of two things will happen, depending on who got hit with it, and your DM.

Target was an NPC, and DM is using the default rules

They are dead.

Per the Basic Rules, the standard for NPCs reaching zero hitpoints is this:

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.

Being knocked unconscious

Target gets to make Death Saving Throws.

This is the default for Player Characters, and an option for NPCs, again per the Basic Rules:

Mighty villains and special nonplayer characters are common exceptions; the DM might have them fall unconscious and follow the same rules as player characters.

In this case, the character falls unconscious, and still makes an Intelligence Saving Throw

"Unconscious" is a condition that is specified in the rules, and here's what it does:

• An unconscious creature is incapacitated, can't move or speak, and is unaware of its surroundings.
• The creature drops whatever it's holding and falls prone.
• The creature automatically fails Strength and Dexterity saving throws.
• Attack rolls against the creature have advantage.
• Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.

And here is what Incapacitated does...

• An incapacitated creature can't take actions or reactions.

Note that it calls out that an Unconscious creature automatically fails Strength and Dexterity saving throws, but it does not state that they fail Intelligence Saving Throws.

So, since none of the conditions applying to the creature make it auto-fail Intelligence saves, it must make the save as normal.