With hellish rebuke I can take 1 reaction on being damaged by "a creature" within 60 feet to do damage back at him. The Sorcerer Twinned Spell allows me to target a 2nd creature within range. The requirements are only "targets one creature" and doesn't have a range of self. Reading it as RAW so far Twinned Spell would work on Hellish Rebuke but then the description for Hellish Rebuke states "creature that damaged you is momentarily surrounded by hellish flames". Well the Twinned 2nd creature didn't do damage to me what should I do now?
Twinned Spell doesn't work with Hellish Rebuke
Well, technically it does, but only if you are somehow damaged by two creatures at once. Like you note, Twinned Spell only requires that the spell you twin targets one creature and doesn't target yourself, and Hellish Rebuke certainly qualifies. However, all of the targets of the Twinned spell need to be legal targets. The only legal target for Hellish Rebuke is the creature that damaged you, so your second target from the Twinning is wasted.
If you were somehow damaged by two creatures simultaneously, you could twin a Hellish Rebuke, but it wouldn't work otherwise. Also, I'm not certain that the system technically allows 2 creatures to deal damage simultaneously.
1\$\begingroup\$ Hellish rebuke does not specify a time frame for the damage. Just "the creature that damaged you is momentarily surrounded by hellish flames" As long as two creatures have hit you, and the spell is cast after the 2nd one does, it should have two valid targets. \$\endgroup\$– GcLMar 27, 2019 at 13:11
7\$\begingroup\$ @Grosscol Yes, it does. The reaction trigger is in response to taking the damage. If you don't act on the trigger immediately (i.e. as soon as you take damage), you don't get to react to it later. \$\endgroup\$– T.J.L.Mar 27, 2019 at 13:49
\$\begingroup\$ @T.J.L. the content of the spell does not specify that. If two creatures hit you, and you take your reaction the second time, the first creature still is still a valid answer to "which creature damaged you?" The spell doesn't specify it has to be the most recent. \$\endgroup\$– GcLMar 27, 2019 at 17:30
1\$\begingroup\$ @Grosscol if you fundamentally disagree with my answer, you should make a new answer yourself. Your comments here are pretty close to answering in the comments, since you're significantly disagreeing with my interpretation here, and not just clarifying. \$\endgroup\$ Mar 27, 2019 at 17:38
\$\begingroup\$ If two creatures prepare an action to hit something when a condition is met, then they can hit simultaneously \$\endgroup\$ Nov 25, 2020 at 12:41
No, it doesn't work.
Hellish Rebuke can only target a creature that damages you. It is not possible for two creatures to damage you at once in 5e.
Let's say you move out of a square threatened by two other creatures, and both use their reactions to attack you, hitting you and dealing damage. Even in this case, the DM determines the order in which these attacks are resolved; one creature hits you, then the second creature hits you.
So even if you choose to exercise your reaction to cast Hellish Rebuke after the second creature hits you, the trigger you are using for Hellish Rebuke is being hit by the second creature -- for the purposes of the spell, THAT creature is the creature that damaged you; the prior attack had already resolved prior to the spell trigger.
Can Metamagic apply to Hellish Rebuke? Of course! You could Empower the spell, for instance, to try to increase the damage dealt back to them. But you can't Twin the spell, because there is no valid second target when the spell condition is triggered.
Yes; you can use Metamagic on Hellish Rebuke
PHB pg101 : "...you can use only one Metamagic option on a spell when you cast it, unless otherwise noted..."
This is saying that when you cast a spell (whether or not it's your turn as the Spellcasting action, or your enemies turn as a Reaction, you are still casting a spell), you may enhance it with one of your chosen Metamagic enhancements.
PHB pg102: "...when you cast a spell that targets only one creature and does not have a range of 'self' ..."
This is saying that as long as the spell you are casting targets one creature, and YOU are not that creature (It can be an enemy creature or an ally creature), you may enhance that spell with Twinned Spell, allowing you to choose a second creature (the same one or a separate one) for the target of the spell.
PHB pg250 Hellish Rebuke Range: 60ft "...you point your finger and the creature who damaged you..."
The important parts of any spell are the first and last parts. The parts that tell you it's casting time, target/range, duration, and components; followed by the damage structure and type. Anything in between is flavor text. If you wanted hellish flames to erupt from the ground and be purple? That's fine. If you wanted to shake your fist at the creature instead of point? That's fine. If you wanted to point at the first creature and then point at a second creature after you Twin Spell? That's fine too. The 'you point at the creature who damaged you' isn't a rule, it's just elaborate flavor text.
So yes, Hellish Rebuke is a spell that the user casts due to the triggering event. He's still casting it, it's still subject to meta magic enhancement, and you may choose a second target for the cloned spell; the same target or a different one regardless of if it attacked you or not. Why? Because when you Twin Spell a spell, you are effectively creating a copy of the same instance of magic you cast. Same goes for using Twin Spell on a spell such as Witch Bolt. You clone it, may choose a second target, and may concentrate on both each turn as it's a copy of a single instance.
3\$\begingroup\$ -1; this answer ignores the targeting restrictions built into the spell in question. \$\endgroup\$– T.J.L.Mar 27, 2019 at 13:07