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:
- +14: Fighter level 20, with 20 Strength and a +3 Weapon.
- +16: Barbarian level 20, with 24 Strength and a +3 Weapon.
- +16: Monster CR 20+ with 30 Strength.
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:
|Chance to Hit
||Max Target AC
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 has the following first-level feature:
Beginning at 1st level, while you are wearing no armor and not wielding a shield, your AC equals 10 + your Dexterity modifier + your Wisdom modifier.
Which may be combined with the following item:
Bracers of Defense
While wearing these bracers, you gain a +2 bonus to AC if you are wearing no armor and using no Shield.
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:
- Light Armor: 12 + Dexterity Modifier, maxes out at 17, or 20 (+3)
- Medium Armor: up to 15 + min(2, Dexterity Modifier), maxes out at 17 or 20 (+3).
- Heavy Armor: up to 18 or 21 (+3).
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.
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:
- Armor give an AC formula.
- Shield give a good AC bonus (+2 to +5), at the cost of an occupied hand.
- Miscellaneous items give a small AC bonus (+1 to +3), at the cost of Attunement.
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.