Can Hellish Rebuke be trigged by a damaging effect that also causes a condition such as "paralyzed" or "incapacited"?

Question

Similar to a question posed here: Can you cast Hellish Rebuke as a reaction to damage that reduces you to zero hit points?

However, I am wondering what should happen when a condition is applied as a result of a damaging effect that forces a creature to make a saving throw. Specifically when:

1. a saving throw is triggered after the damaging effect.
2. a saving throw is triggered along with the damage.

The Mind Flayer has actions that fit both scenarios. The first is its "tentacles."

1. When a Mind Flayer hits a medium or smaller creature with its tentacles, it deals 2d10+4 psychic damage. The creature is then grappled and

[...] must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends.

A stunned creature is incapacitated. The incapacitated condition reads:

An incapacitated creature can't take actions or reactions.

In this case, is the damaged creature allowed to use its Hellish Rebuke reaction prior to making the saving throw or must the effect of the saving throw be resolved prior to the reaction?

Hellish Rebuke's trigger:

1 reaction, which you take in response to being damaged by a creature within 60 feet of you that you can see.

2. The Mind Flayer's "Mind Blast" will force each creature in the affected area to succeed on a saving throw or

take 4d8+4 psychic damage and be stunned for 1 minute.

Here, the damage and the condition are both applied after the saving throw. Could Hellish Rebuke be triggered in this instance?

Answer 1

Hellish Rebuke cannot be used here

In both cases, the tentacles Melee Weapon Attack, and the mind blast, the attack deals damage and forces a saving throw. The Dungeon Master’s Guide says under "Saving Throws" on page 238:

A saving throw is an instant response to a harmful effect

That is, making a saving throw does not introduce any time delay.

Tentacles

The Monster Manual says under "Melee and Ranged Attacks" on page 10:

Hit. Any damage dealt or other effects that occur as a result of an attack hitting a target are described after the “Hit” notation.

Tentacles. Hit: 15 (2dl0 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned

So the saving throw and damage occur as a result of the attack hitting the target, at the same time.

Mind Blast

Mind Blast (...). Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8 + 4) psychic damage and be stunned for 1 minute.

Damage and stun both are the effect of the failed saving throw, and happen at the same time.

There is no concept of time delay between the damage and other effects of these attacks. There is no "and then", or "and after that". Both the damage and the debilitating effect are combined by "and", they are happening at the same time. If there was a delay between the attack and the other parts of the effect, the description would say so. Compare this for example with the effect of the Behir's swallow attack that has language about the target taking damage at the start of the Behir's turn.

By the time you could cast Hellish Rebuke, you are already unable to act

Both the paralyzation and the stun effect cause the incapacitated condition which states that the incapacitated creature can’t take actions or reactions.

In general, reactions only resolve after the triggering effect finishes. The DMG says on page 252 under "Adjucating Reaction Timing":

If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes

Hellish Rebuke's casting time is:

1 reaction, which you take in response to being damaged by a creature within 60 feet of you that you can see

As explained in the linked question, you have to already have taken the damage for this trigger condition to be fulfilled, and will at the same time have suffered the instant effects of any failed saving throws. Unfortunately for you, if you failed your save, by the time you could cast the spell, you are incapacitated and unable to take reactions. Therefore, you cannot cast Hellish Rebuke.

(If you made your saves and did not drop to 0 hit points from the attack, you could cast Hellish Rebuke.)