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?