Can one use a hellish rebuke as a reaction after taking damage that would knock one unconscious?
A friend and I fought during one game and I hit him, so he would have lost, but used his reaction to finish me, too. (It was awesome :D)
Unfortunately, as awesome as this sounds, by the rules, it doesn't work. Hellish Rebuke is a reaction that you take
in response to being damaged
Not "in response to being hit", or "in response to being attacked". You actually have to take damage to use it.
Whenever a creature takes damage, that damage is subtracted from its hit points.
You haven't taken the damage until you've subtracted it from your hit points. If that puts you on 0 hit points, then
you either die outright or fall unconscious
At which point, you are incapacitated and can't use reactions.
The rules are (my emphasis):
Hit Points (PHB p.196)
Whenever a creature takes damage, that damage is subtracted from its hit points. The loss of hit points has no effect on a creature’s capabilities until the creature drops to 0 hit points.
Unconsciousness (PHB p.197)
If damage reduces you to 0 hit points and fails to kill you, you fall unconscious.
Hellish Rebuke (PHB p.250)
Casting Time: 1 reaction, which you take in response to being damaged by a creature within 60 feet of you that you can see
Reaction (PHB p.190)
A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s.
There are two possible sequences here:
damage (trigger) -> reaction (Hellish Rebuke) -> subtracted from hit points -> unconscious
(damage -> subtracted from hit points -> unconscious [all as a single event]) (trigger) -> can't react because unconscious
For mine, a reaction is an instant response to the trigger, and instant means instant; it interrupts the normal sequence of subtracting damage from hp and falling unconscious (or dying) so the first option is the one I think works.
Notwithstanding, awesome should always beat the rules.