How does Protective Bond interact with Armor of Agathys?

Question

Armor of Agathys

A protective magical force surrounds you, manifesting as a spectral frost that covers you and your gear. You gain 5 temporary hit points for the duration. If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage.

Protective bond

Beginning at 6th level, the bond you forge between people helps them protect each other. When a creature affected by your Emboldening Bond feature is about to take damage, a second bonded creature within 30 feet of the first can use its reaction to teleport to an unoccupied space within 5 feet of the first creature. The second creature then takes all the damage instead.

If a peace domain cleric is about to take bludgeoning, piercing or slashing damage from a melee attack and a warlock, who has armor of agathys, uses their reaction to teleport in due to protective bond, does armour of agathys activate and damage the attacker?

the second creature takes all the damage instead

in what way does the second creature take that damage?

Is the damage being redirected from the attacker to the second creature (so armor of agathys would activate)? or is protective bond absorbing the damage and outputting it as magical bludgeoning, piercing or slashing spell?

Answer 1

Only damage is redirected, not the attack

The key lies in the sentence you highlighted :

The second creature then takes all the damage instead.

Any side effects of the attack still affect the original target, which is the one hit by the attack. The second creature takes the damage, but is otherwise unaffected by any other effects the attack might carry.

What is the rebound damage exactly?

Deciding exactly how the rebound damage works in terms of applying resistances, what damage type it is and other specifics still seems to be an undecisive topic. Here are a few questions that treat some of those topics and arguments for each sides :

• Can you know the damage before teleporting with Cleric Peace Domain Lvl6 Protective Bond? (the accepted answer does a great job at taking on difference damage sources)
• Warding Bond: What is the Order of Operations for calculating cleric damage taken? (another take at how to apply damage resistances/weaknesses)
• Does Warding Bond carry over damage type? (concerning the damage type of the rebound damage)

Back to Armor of Agathys

Let's look at the trigger for the cold damage from Armor of Agathys :

If a creature hits you with a melee attack while you have these hit points, the creature takes 5 cold damage.

Since the second creature is not hit by the attack, this damage does not trigger the cold damage from Armor of Agathys. The temporary hit points may still soak the damage, however.

What about the other way around?

If the first creature (the one taking the hit) has an active Armor of Agathys, even if they avoid the damage, they are still hit by the attack. This means that the attacker will take the cold damage from Armor of Agathys, even though the armor's temporary hit points will not be reduced.

To put it simply, the attack is hitting the armor, triggering its effect, but all damage is redirected to the second creature.

This actually works fairly well, since you may trigger the armor's damaging effect without actually damaging it, meaning you may get more value out of it if you get hit multiple times. This, of course, assumes the enemy is stupid enough to attack the same person many times without reacting to the fact they're taking damage whenever they do, without damaging their intended target.

Answer 2

Matthieu has a good answer explaining how the rules work so this is less RAW and more how I interpret the flavour and intention of these abilities, and as a DM I think it is more important to preserve flavour and verisimilitude than it is to stick to RAW when it sometimes isn't intuitive.

The premise of protective bond is that you teleport to defend an ally and push yourself in the way, so you take the blow for the person you are defending. It isn't strictly written that way, but that is easy to explain when you realise that damage is the most common effect of being hit. The rules are not meant to be lawyered and taking a rule from one book and putting it with a rule from another book to find discrepancies, they just aren't written or proofed for that.

In certain edge cases, where there are additional effects upon the hit, such as being pushed (or swallowed!), it doesn't make sense that one target can get the effect but the other takes the damage. The damage and the effect make more sense as being part of the same thing.

So I rule protective bond as the defender pushing themselves in the way of the blow at the last moment, and taking the blow. I just can't see a way that makes sense in spirit or narrative (unless you handwave it with "its magic") to rule it any other way.

Therefore I think, going from the how to play section in the PHB:

The DM describes the environment: "The large hairy slime reaches out with its prehensile ear to slap you"

The players describe what they want to do:"I want to use protective bond to protect my friend, nobody gets to kill them but me!"

The DM narrates the outcome: "You notice the ear has a vicious looking piercing that could do some real damage so you use the bond of frenemy-ship to intercept, teleporting across the battlefield to push your friend out of the way, taking the damage yourself"

I therefore would rule it as the full blow hitting the new target, and so Armour of Agathys would trigger, as would any other effects like pushback, being swallowed etc.

The only time I would really allow a straight damage transfer is when the intervening force is clearly magic, like warding bond.