Last night, an Owlbear attacked us. It was injured, then attacked my Warlock. Obviously it uses its Multiattack action (MM, pg. 249):
Multiattack. The owlbear makes two attacks: one with its beak and one with its claws.
After the first attack, I wanted to cast hellish rebuke on it, knowing that it might kill it before it unleashes its second attack.
Hellish rebuke says (PHB, pg. 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
…
You point your finger, and the creature that damaged you is momentarily surrounded by hellish flames.
The trigger is being damaged, so I should have been able to cast this after the first attack but before the second. At least that's how I understand it.
Because my DM rolls both attacks and damage at the same time for expediency, he argued that the spell would be cast after the Owlbear finished attacking, because it hadn't yet completed its action (the Multiattack action), and reactions come after their triggering action. In other words, I would have to take both attacks before I could react.
Who was right?
Best Answer
Reactions can interrupt multiattack.
The rules on reactions says:
Emphasis mine.
Multiple attacks on one turn are not simultaneous, as Jeremy Crawford has stated:
Here is the definition of Multiattack (MM, Pg. 11):
Neither the Multiattack definition nor the Owlbear's game statistics say that the attacks occur simultaneously. So in your case, the following sequence occurs: