17 Mar Armor of Agathys 5e D&D Guide 
A spectacular defense spell, Armor of Agathys absorbs and deals damage. It is widely considered one of the best early warlock spells.
The rules for Armor of Agathys are found in Players Handbook on page 215.
Armor of Agathys 5e
Abjuration 1st Level
Casting Time: 1 Action
Components: V, S, M (A cup of water)
Duration: 1 Hour
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.
At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, both the temporary hit points and the cold damage increase by 5 for each slot level above 1st.
The rules for Armor of Agathys give a good description of the visual components of the spell and a straightforward explanation of how it works. The interesting thing with this spell is that it specifically states you and your gear, meaning this spell works in addition to any other armor or protections you have. The description also provides a good amount of leeway in terms of flavoring potential.
Is Armor of Agathys Good?
Armor of Agathys (AoA) is part defense spell and part damage spell. This sets it apart from other spells such as Shield and Mage Armor. AoA provides temporary hit points for the caster but also deals damage to a melee attacker.
It is important to note that the spell only activates on a hit from a melee attack. This makes it a much better spell for a warlock who plans to be up in the fray compared to a ranged character.
With the idea that a sword-wielding warlock is going to benefit more from the spell, AoA is a great choice. It is available right away, and the benefits of casting all spells at max level make the spell well worth it. At a 3rd level cast, it grants 15 temporary hit points and does an automatic 15 damage to the attacker while any of the hit points remain.
The Armor of Agathys spell was almost exclusively built for a Pact of the Blade warlock. It requires the character to take melee damage to activate. There are few casters who are going to be in melee range more than the blade warlock. Yes, they have Eldritch Blast, but when they can attune to magic pact weapons, Eldritch Blast can quickly lose its luster.
Is Armor of Agathys a Concentration Spell?
As defined in the spell, Armor of Agathys does not require concentration. It lasts for an hour when cast. This allows the warlock to utilize other spells (like Hex) while the AoA is up. The duration is long enough for the spell to be cast in advance of real danger, which is a benefit.
Armor of Agathys does not allow for the damage to be withheld. If for some reason, you were trying to save the damage dealt for a specific enemy and were hit by a different enemy, you cannot forgo the damage effect to use later.
What does Armor of Agathys do?
To thoroughly understand the benefits of the Armor of Agathys spell, it should be broken down into benefits. First, the temporary hit point benefit from the spell, and second the damage capability of the spell. Keep in mind this spell is designed for melee-heavy warlocks. Ranged attack-based characters can probably find a better use of the slot.
First, AoA grants temporary hit points. The caster gains 5 HP per spell level used. Since warlocks always cast at the highest level, it is easy to see what the benefits of the spell will be. At higher levels, a warlock should gain enough hit points to withstand a few attacks, with these hit points meaning more damage will come out of the spell.
Second, as long as the caster has some of the temporary hit points remaining, they continue to deal the damage from Armor of Agathys. Using Armor of Agathys at 3rd level gives the caster 15 temporary hit points. If an enemy hits the caster for 3 damage, the enemy takes 15 damage. At this rate, the enemy will hit 5 times before the temporary hit points are gone. During that time, the enemy would take 5 x 15 in damage for a total of 75 damage.
Armor of Agathys, like many of the spells in the game, is situational. It is geared toward a warlock who doesn’t mind getting in close and mixing it up. It also comes early on as a 1st level spell, which makes it a bonus for any multi-class combo that might be interesting.
Any spell that gives a healing or defensive bonus, in addition to granting damage to an attacker, has to be considered. There are few dual-purpose spells in the game, and not many can be as flavorful as Armor of Agathys.