Can a reaction interrupt multiattack?

Question

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?

Answer 1

Reactions can interrupt multiattack.

The rules on reactions says:

A reaction is an instant response to a trigger of some kind, which can occur on Your Turn or on someone else’s. [...] If the reaction interrupts another creature’s turn, that creature can continue its turn right after the reaction.

Emphasis mine.

Multiple attacks on one turn are not simultaneous, as Jeremy Crawford has stated:

Q: Eldritch Blast: are the attacks resolved in parallel or sequence? Do you have to pick all the targets first before rolling?

A: Multiple attacks on the same turn aren't simultaneous, unless a feature or spell says otherwise.

Here is the definition of Multiattack (MM, Pg. 11):

A creature that can make multiple attacks on its turn has the Multiattack action. A creature can't use Multiattack when making an opportunity attack, which must be a single melee attack.

Neither the Multiattack definition nor the Owlbear's game statistics say that the attacks occur simultaneously. So in your case, the following sequence occurs:

1. The Owlbear's first attack hits you.
2. You use your reaction to cast Hellish Rebuke in response to being damaged.
3. The Owlbear continues with its multiattack (unless it is now dead).

Answer 2

Your reaction happens immediately.

You were hit by an attack. The term multiattack directly implies more than one attack. With the owlbear as the example, each of its attacks have their own attack rolls. This makes them extremely well defined events separated by specific rolls.

As with the shield spell it happens in response to the first triggering event (which in this case would be the first time you take damage).

An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.

• which you take when you are hit by an attack or targeted by the magic missile spell

Same as with Misty Escape. You vanish in a puff of mist in response to harm. The triggering event is the first individually defined bit of damage, not an entire barrage of attacks.