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
Not "in response to being hit", or "in response to being attacked". You actually have to take damage to use it.
You haven't taken the damage until you've subtracted it from your hit points. If that puts you on 0 hit points, then
At which point, you are incapacitated and can't use reactions.
The rules are (my emphasis):
Hit Points (PHB p.196)
Unconsciousness (PHB p.197)
Hellish Rebuke (PHB p.250)
Reaction (PHB p.190)
There are two possible sequences here:
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.