[RPG] the lowest possible AC

armor-classclass-featurednd-5eoptimization

This is a direct, somewhat silly, opposite question to What is the highest possible AC? .

I’m wondering what is the lowest possible Armor Class a Player Character can get at any point in time, “helped” by any official relevant monstrous feature, magic item, spell, etc.

So far, the lowest I’ve obtained is a 0, provided that the player character is a (Monk or Barbarian) with both Dexterity and (Constitution or Wisdom) rolled as a 3 (-4) without any racial bonuses to said stats and that said character is affected by a Slow spell.

Can you go even lower than that ?

Best Answer

The lowest reasonably achievable Armor Class is -7

The Character

The character, lets call him Colby, to accomplish this absurdly low AC is a character with rolled ability scores resulting in 3 Dexterity, 3 Charisma, and either 3 Wisdom or Constitution.

If 3 Wisdom, Colby requires at least 1 level of Monk, and 3 Constitution requires at least 2 levels of Barbarian.

The Gear

Colby needs these magic items:

  • Chromatic Dragon Mask (found in an adventure module below)

Hoard of the Dragon Queen and Rise of Tiamat

  • Book of Vile Darkness
  • Wand of Orcus, Sword of Kas, or Orb of Dragonkind

Each of the artifacts have major detrimental properties (the Book of Vile Darkness has 2). One option of which is:

$$\begin{array}{|c|c|} \hline \text{}&\text{When you become attuned to the artifact,} \\ \text{76 ‒ 80}&\text{one of your ability scores is reduced by 2} \\ \text{}&\text{at random...} \\ \hline \end{array}$$

These reductions need to affect Dexterity, Charisma, and whichever of Wisdom and Constitution is low.

The Calculation

With the gear mentioned above, and the slow spell, you get this Armor Class:

  1. With Dexterity and Constitution or Wisdom at 1 are AC with Unarmored Defense is 0

While you are not wearing any armor, your Armor Class equals 10 + your Dexterity modifier + your Constitution modifier (Wisdom modifier for monk).

  1. The Chromatic Dragon Mask has the following effect:

Draconic Majesty. While you are wearing no armor, you can add your Charisma bonus to your Armor Class.

This results in another -5 to AC, for AC of -5 so far

  1. The slow spell.

An affected target's speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can't use reactions.

Finally, with 2 less AC, we are at an AC of -7

Strict RAW

A strict adherence to rules-as-written provides another answer:

There is no lower bound

You can reduce armor class infinitely with a special procedure.

The Garb

The attire for the infinitely regression of AC is one of the following armors:

  • Chain Shirt
  • Scale Mail
  • Breastplate
  • Ring Mail

The Monsters

There are two monsters required to reduce your armor class forever.

  1. The demon lord, Juiblex

Juiblex has an ability called Eject Slime which says:

Unless the target avoids taking any of [the acid damage from this attack], any metal armor worn by the target takes a permanent −1 penalty to the AC it offers, and any metal weapon it is carrying or wearing takes a permanent −1 penalty to damage rolls. The penalty worsens each time a target is subjected to this effect. If the penalty on an object drops to −5, the object is destroyed.

  1. Any number of ooze monsters including the Gray Ooze whose Pseudopod attack includes:

if the target is wearing nonmagical metal armor, its armor is partly corroded and takes a permanent and cumulative −1 penalty to the AC it offers. The armor is destroyed if the penalty reduces its AC to 10.

The Procedure

The first step is to get hit by Eject Slime 4 times (3 times for Chain Shirt). This reduces the armor class granted by the armor to 10, but does not destroy it as that would require a 5th reduction from Juiblex:

If the penalty on an object drops to −5, the object is destroyed.

Then you just need to get repeatedly punched by the Gray Ooze. After the first Pseudopod, the armor bonus is reduced to 9, which does not meet the clause:

The armor is destroyed if the penalty reduces its AC to 10.

... since it specifies exactly 10 (not less than 10). Then you can repeat this procedure and the armor gets progressively higher penalties as long as you can continue to survive the attacks (which the right character can survive forever).

Now, clearly this is nonsensical, as acid shouldn't be able to suddenly make your armor class lower than if you weren't wearing armor at all, but that's how the rules work together as written.

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