How does concentration work when wild shape is ended through damage?

Question

Consider the following situation (round numbers chosen for easier calculations):

• Bob is a human druid.
• In caster form Bob has a +0 CON modifier and 100 current HP.
• Bob is concentrating on a spell.
• Bob is wild-shaped. His current beast form has a +5 CON modifier and a current HP of 30.

What concentration checks are required (including DC and modifier used on the roll) if:

1. Bob is dealt 30 damage
2. Bob is dealt 60 damage

As far as I know concentration is not automatically broken in either situation because although the druid counts as hitting 0 HP he is never unconscious.

Answer 1

What the Rules Say

Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher.

Means that when you have taken damage. This is different than other effects that happen "when you are hit" like Shield:

Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell

This difference, as evidenced by the two difference sets of language, mean that the order is:

1. You are hit
2. Reactions and effects that trigger on hit
3. Take damage (HP changes)
4. Reactions and effects that trigger on damage

this is covered in a question about Hellish Rebuke: Hellish Rebuke reaction with zero hitpoints

In This Case

The above applied means that the druid who takes damage that drops their wild shape form would use their own, not the beast form's, CON Save for the check. Both would be Bob's Con.

So following the list...

30 damage

1. Hit happens.
2. No on hit events.
3. Damage happens, reduced to zero, form dropped.
4. Concentration check against Bob's CON Save vs DC 15.

60 Damage

1. Hit happens.
2. No on hit events.
3. Damage happens, reduced to zero, form dropped, Bob takes 30 rollover damage.
4. Concentration check against Bob's CON Save vs DC 30.

Extra example to prove case

There is an class feature in Barbarian class:

Retaliation. Starting at 14th level, when you take damage from a creature that is within 5 feet of you. You can use your reaction to make a melee weapon attack against that creature.

Given this, following the idea Miniman posted that taking damage effects happens before the damage. The Barbarian is reduced to zero, and then gets a reaction (after being knocked unconscious) that allows him to make another attack. When it is clear that the barbarian would be eating dirt not still swinging.

Answer 2

This is pretty simple, actually.

Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher.

You make a Constitution saving throw "whenever you take damage". So when you take 30 damage, you make a Constitution saving throw. Since you make this when you take the damage, you use the stats you have when you take the damage. So it will be a DC 15 Constitution saving throw which you make with a +5 bonus.

The same reasoning applies to the 60 damage case. So when you take 60 damage, you make a DC 30 Constitution saving throw using a +5 bonus. There is a caveat, though.

When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. For example, if you take 10 damage in animal form and have only 1 hit point left, you revert and take 9 damage.

This suggests that the 30 carryover damage will count as being taken, so you'll have to make another Constitution saving throw, this time at DC 15, using your own +0 modifier.