Am I misreading you, or do you somehow have the idea that Barrier Tattoo doesn't let you use a shield? Because it specifically says you can. Also there's a big difference between having AC 18 and having AC 18 for one round when you spend a spell slot.
Anyway, you're talking white room theorycraft where you have free picks from the entire magic item list, not what PCs might be happy to find as loot. But even if we assume you can build your character with the certainty of getting this magic item... you're looking at it from the wrong end. It's not an item for boosting the AC of a melee focused character. They tend to have pretty good AC naturally. It's an item for boosting the AC of characters who don't typically have good AC because they don't come with heavy armor prof or high Dex.
I understand that, they also don't generally come with Shield proficiency. In fact of the "most likely to want this instead of armor" list I can think of.. only the Barbarian and Hexblade can use shields.
But, I think we are talking past each other to a degree. I'm looking at this as "how does this stack up to other items of similar values"
The Uncommon is invisible studded leather that does not require proficiency. Essentially any class I can think of can do better than this. Maybe Sorcerers would like it, but it is really not worth an attunement slot.
The Rare (taking up an entire limb for tattoo) is invisible half-plate, also does not require prof and does not give stealth penalty. Sure, for a wizard with a 14 dex, suddenly having a 17 AC is pretty nice. Certainly wouldn't turn my nose up at it if I had a free attunement slot. For any non-caster class who actually has the armor... just better to have the armor in most cases. Because, if we assume you get this Rare item that costs attunement, maybe you could instead get a different rare item that costs attunement. And that item is likely to offer you a better bonus, like a Ring of Spell Storing for a random example. That item can offer a lot of flexbility for a caster.
For example, a Monk with an 18 Dex and 16 Wis (not unreasonable) has the same AC that this tattoo provides already. Getting a Rare Ring of Protection gives them better AC and Better saves. 14 Wis and it is equivalent AC and better saves.
Also, in my estimations, not penalizing stealth is only as valuable as your stealth score is anyways. If you only have a +2 for stealth, rolling normally or at disadvantage isn't making a massive difference, because you are likely not stealthing anyways.
And we run into the same thing with the Very Rare (takes up entire torso or two limbs) invisible plate. Again, for a monk or barbarian, hardly worth the attunement slot. For a caster like a wizard, it could be quite nice, but it is a Very rare item. This is supposed to be equivalent to a Staff of Power, which gives +2 to hit, damage, stacking +2 Ac, and 20 charges for casting a variety of spells.
Sure, not all items of the same rarity are made equally, but "make my wizard have armor" is a meh item at best. Especially since, lest we forget all of our magic items, Elven Chain is a rare armor that does not require attunement. Everyone is proficient with it and gives a 14+ max 2 dex, so it is quite comparable to the first two levels of this tattoo in terms of benefit without preventing you from using other items.
I'm not so sure it needs the nerf. It's an excellent spell, don't get me wrong, but it has some major drawbacks compared to
holy weapon:
- You can't put it on somebody else. Holy weapon can be cast on the deadliest weapon-wielder in the party; give it to a PAM fighter or a frenzying berserker, and watch them go to town. Spirit shroud can only be used by you, the caster, which means you have to a) be an effective melee combatant and b) have 5th-level spell slots. This is not impossible, but you're not going to get as much mileage out of it as a dedicated martial.
- It's a concentration spell that forces the spellcaster into melee. The risk of losing concentration is high. With holy weapon, you have the options to put it on someone else or use it with a ranged weapon, allowing you to stay in the back rank.
- The duration is capped at 1 minute, so even if you do hold concentration, you will never get more than one combat out of it. Holy weapon, with a 1-hour duration, can be sustained through multiple battles.
I had forgotten about the hour long duration of Holy Weapon, that is a really good and salient point.
Also the fact that this is a self spell instead of a touch spell, but I think that part was understood.