I recently had an idea for a new item in 5e. This item is a cloak that lets you add your Wisdom modifier to your AC (requires attunement). I was thinking of making it a rare magic item, and I was looking for advice.
Is this item overpowered as a rare? Properly balanced? Underpowered? If it is overpowered, what are some changes I can make so it can stay the same rarity and keep its core concept?
The rarity is based on the fact that an amulet of health, which increases the survivability of non health-based classes a lot, is rare. The amulet of health also only needs to be worn on the neck, while this cloak has to be worn in place of armor, an extremely common item to find enchantments on. Another item that adds the same amount of AC (overall) is a +3 shield, which is very rare; however, that takes up an uncommon slot, the shield slot, so I think the cloak is sufficiently less powerful to decrease the rarity. I am looking for advice on changes to make if this item is overpowered, not to scrap the idea entirely.
Note: I have made a less powerful item that is designed based of advice from this question. I am asking for advice on balance here.
Best Answer
On Bounded Accuracy and Armor Class
Before attempting to judge the item, it is important to first understand AC (Armor Class), and the effect of AC on play.
AC is the "threshold" to which AR (Attack Roll) are compared to, therefore any discussion about AC must be calibrated by the distribution of AR values of the opponents of the creature, so as to be able to determine the chances that said opponents will hit or miss.
Distributions are complicated, so I like to look close to the extreme for a quick ballpark: how will it play at level 20?
Heavy hitters sample:
There are (rare) ways to get better AR bonuses -- using miscellaneous bonuses -- but those are close to the top AR bonuses that anyone will ever face.1
From there, we can determine the chances to hit for a number of ACs, though I'll reverse it to determine the target AC for a few set chances to hit.
Let's assume the opponent is the Fighter (+14), without Advantage:
So, if you want the Fighter (+14) to hit 50% of the time (ie, on a roll of 11+), then the opponent must have an AC of at most 25.
1 Why are the AR bonuses so "fixed"? Because of Bounded Accuracy, which vastly reduced the dynamic range of all roll modifiers. See questions such as Why is Bounded Accuracy called Bounded Accuracy? for more information on Bounded Accuracy itself, and the problems it aimed to solve.
Evaluating the Item
From there, in order to evaluate our item, we need to evaluate whether it "breaks" the typical range of AC values. As a DM, you may be in position to prevent the breakage, of course, however the more potential for abuse, and the more intervention needed, the less balanced the item itself is.
The Monk
The Monk has the following first-level feature:
Which may be combined with the following item:
An optimized Monk is likely to have reached 20 Dexterity and 20 Wisdom by level 20, so they would be looking at an AC of 22 with the Bracers, which is still reasonable as per our computations above.
If instead of the Bracers they use the cloak, they immediately get an AC of 25, and if they add the Bracers, an AC of 27.
For a permanent AC -- not the result of a short-term buff -- this is very strong. Furthermore short-term buffs can bolster it temporarily.
The Cleric, the Druid, and the Ranger
With a single 1-level dip into Monk, any of those characters can pull the same shenanigans, and being Wisdom-based, they have little opportunity cost for doing so.
The Martial Classes
The typical formulas for martial classes ACs are:
Replacing the armor with your cloak, supposing the character has a Wisdom of 20 otherwise why bother, allows the character to reach an AC of 20.
The cloak is thus equivalent to a Light Armor +3 or Medium Armor +3 in terms of power for those characters.
Judgement: This item is Very Powerful, it may easily equal +3 armors, and with the cost of a single level Monk-dip straight eclipse them.
Recommendations
As has been seen with the Monk, and assorted Wisdom-based casters, a straight bonus to AC is dangerous for balance.
This is the reason that there are 3 class of items granting AC in 5e:
Since your cloak replaces an armor, it should give an AC formula.
For example, your cloak could give an AC of 12 + Wisdom Modifier as an Uncommon Item, and rise all the way to an AC of 15 + Wisdom Modifier as a Legendary Item, matching a Light Armor AC calculation.