I realize this question was asked some time ago, but the existing answer did not respond to it. The question asked was what advice might be given for constructing a homebrew version of Barkskin that retains the flavor of 3.x but is consistent with 5e rules.
As I am currently dealing with the problematic nature of the barkskin spell in my own campaign, I took to the web for help and have found that there is not much help to be had. While there are lots of responses in the vein of "just use it as it's written", they are not very satisfying to the folks that can't help but ask "why is it written that way?"
Likewise, interpreting (or defending) a rule does not do much to address the fact that it is a poorly written rule. I say it is poorly written because it is internally inconsistent with all the other rules dealing with AC, and because it has in actuality created significant amounts of confusion and frustration among players of the game.
So to answer the original question, here are some suggestions for how to adjust Barkskin to make it work more like everything else in the game already does.
TL;DR is at the end of this post
I think there are 3 threshold questions to be asked:
- Should the spell involve natural armor rules?
- How does the protection granted work with other adjustments to AC (stacking)?
- Should the spell be scalable? (In other words, should it's power increase if cast with a higher level spell slot?)
1. Natural Armor
3.x barkskin affected the target's natural armor, which was one component of total AC, along with armor, shield, deflection, dodge, and enhancements. In it's simplest form, it gave +2 enhancement bonus to natural armor, in the way that one might have +2 armor or shield. In most cases that just meant that it gave +2 natural armor bonus. This bonus was then added to armor, shield, etc to determine total AC.
Of course, this isn't how AC works in 5e. Natural armor is still in the rules, but is is clearly designed as an alternative to (artificial) armor. So, many creatures "have" natural armor, whereas the PCs "wear" armor. They are meant to be mutually exclusive. Because of that, it renders the distinction largely meaningless in game terms (they function in exactly the same way), and it really serves only to show the difference between thick/magical skin on creatures and (usually) metal clothing worn by people.
Given this, it is up to the homebrewer to decide if they want to classify barkskin as natural armor or not, but it is clear that it CANNOT stack with other armor being worn. In a way, I think this is what the designers were going for with the 5e wording of the spell, they just didn't explain it very well.
So "natural armor" or not is up to you, it's all just flavor text. In game terms it replaces worn armor (if any), but only if it is a higher AC than that armor.
2. Stacking - how does it work with other AC modifiers?
In his response on the D&D site Jeremy Crawford doubled down on the spell as written, saying that under the effect of the spell your AC is the higher of whatever your AC is normally OR the number 16. That 16 number cannot be modified in any way. If something adjusts your normal AC to make it higher than 16, you get that instead, but the 16 itself will never change.
While I agree that this is the proper execution of the rule as written, it doesn't change the fact that to many people that rule is unsatisfying. I believe the dissatisfaction comes from two things: it is unsupported by any story-driven rationale, and it is arbitrary. Nothing else in the game works this way with regard to armor class. It is an outlier, and a somewhat non-sensical one when you start playing out scenarios:
- (AC 15) chain shirt and shield
- (AC 16) barkskin
- (AC 16) drop the shield because it doesn't matter
- (AC 17) pick up a +2 shield, because NOW the shield matters (for reasons)
So how do we craft a rule that makes sense? The answer is fairly obvious - just make it work like normal armor. Call it natural armor if you like, but make it fit within the existing rules. In essence, the mage armor spell works like +1 studded leather armor. So make barkskin a slightly better version. Two suggestions:
- Barkskin provides AC 14+dex modifier (max 2) - essentially a breastplate
- Barkskin provides AC 16 (no dex mod) - essentially chain mail
OPINION: I would argue for the breastplate option (although in story terms barkskin is in no way analogous to a breastplate). I think medium armor effects make more sense than heavy armor. If you want heavy armor effects, modify the stoneskin spell in the same way (since normal weapon resistance isn't often that helpful by the time 4th level spells are available anyway).
Once you make this adjustment, everything else works itself out and it fits within the rules, just the way mage armor does. It is sturdier than mage armor, but it has a shorter duration (8 hours to 1 hour - I would remove concentration because that destroys any cost/benefit analysis for using it). This solution also resolves all your issues with alternative AC determinations (like Unarmored Defense). If the target if affected by Barkskin, they are considered "armored", and abilities that require a character to be "unarmored" cease to operate.
I think the last thing to consider is whether to make the spell scalable the way many damage spells are. Again, I see two interesting options:
- An additional +1 to AC for every two spell slots higher (15 at 4th, 16 at 6th, 17 at 8th, all with +2 dex modifier max)
- Removal of dexterity limitation at a higher level, say 5th or 6th (14 + unlimited dex, making it effectively +2 studded leather.
I think either of these works and allows the spell to have some utility as PCs reach higher levels. I think the first one is better because it keeps the flavor of medium armor. If someone wants to use an 8th level spell slot to give themselves a 19 AC for an hour, I don't see any reason why they shouldn't be able to.
So, here's my suggestion for a barkskin spell that reconciles better with existing 5e rules.
Barkskin- 2nd-level transmutation
- Casting time: 1 action
- Range: Touch
- Components: V,S,M (a handful of oak bark)
- Duration: 1 hour
You touch a willing creature. Until the spell ends or the caster uses an action to dismiss it, the target's skin has a rough, bark-like appearance, and effectively gains the qualities of medium armor (AC 14 + dex modifier [max 2], no disadvantage to stealth). If the target does not have medium armor proficiency, they are considered proficient only for the purposes of this spell. If the target is wearing other armor, only the higher of the two AC calculations is used (the effects do not stack). The target of this spell is considered armored and may not use abilities that require them to be unarmored.
At Higher Levels. If this spell is cast using a higher level spell slot, the base AC increases by 1 for every two levels higher (15 AC for 4th level, 16 AC for 6th level, 17 AC for 8th level).