Would an artifact that enhanced proficiency be too over-powered?

Question

I am the DM of 5e, and during my campaigns I try to allow characters to get an artifact — a legendary magical item — as a sort of personal side quest to build their character. This is something that happens over the 'lifetime' of their character. I let them choose from the known ones or imagine one of their own. For custom artifacts I compare stats to known item to make sure it isn't too overpowered, i.e. going beyond even legendary.

Recently I had a player that wanted their character to have a magical item that enhanced proficiency. Because proficiency is such an integral component of the game, I am not sure how this would play out. Would +1 prof be more overpowered than even a legendary, or would it be more like +3 on a weapon? The player was aiming for +2, but I already shut this down. Would love to hear some opinions, especially if you have done this and how it worked out.

The player in question is a bard, but I also want to know as a general reference.

Answer 1

Things that proficiency bonus increases:

1. Saves a character is proficient in
2. Attacks with weapons a character is proficient in
3. Ability checks made with skills or tools a character is proficient in
4. Bards and Fighters have ways of adding half their proficiency bonus to untrained ability checks
5. Bards and rogues (and some feats) can add double proficiency bonus to some checks

How can we simulate this with existing magic items?

• A Stone of Good Luck (uncommon, attunement) gives +1 to all ability checks and saving throws. This is better than a +1 to points 1, 3 and 4
• A +1 Weapon (uncommon) gives +1 to attack rolls with it. This is worse than point 2 in general, but for a character who only uses 1 weapon it's not much of a difference

An item that gave +1 to proficiency bonus would probably have to be at least Rare, given that it combines the properties of two uncommon items (and allows for the use of a separate weapon), but I don't see it breaking anything. In a few cases it will add two points to a skill (point 5); this is nothing to be sniffed at, but if the item you're crafting is meant to be Legendary, you're still well within bounds of that.

Spellcasters and proficiency boosts

Spellcasters with a +1 to proficiency get a +1 to their attack rolls and to their save DCs. Increased save DC is a surprisingly thorny issue, but in the worst case (though I'd argue that it's not really any worse than a +1 to hit) we can simulate it with a Very Rare item. Unless you're regularly casting with two different stats, the +1 from a stat-increasing tome is much better than this, since it applies to some ability checks and saving throws also, and is not losable.

Is it overpowered? No.

Our final item is generally worse than a Very Rare and two Uncommons (you're unlikely to be making full use of both the weapon boost and the spell boost), which makes it hard to argue that it's overpowered as a Legendary.

Answer 2

It is acceptable for a legendary item.

There is precedent in the SRD:

Ioun Stone of Mastery (Legendary). Your proficiency bonus increases by 1 while this pale green prism orbits your head.

Now, an Ioun Stone can be thwarted by an enemy who succeeds in snatching it away from you by making a successful Attack roll against AC 24 or a successful DC 24 Dexterity (Acrobatics) check, but this is balanced out by the fact that you have made your item an artifact, an item which is usually a cut above the rank and file list of magic items.

Answer 3

+1 to proficiency is fine as Legendary

Examining it alone, a +3 weapon means about +55% increase in DPR, while being only very rare.
Increasing the proficiency by 1 is not even 10%. It adds to some saves and skills besides attacks too, but it still pales in comparison.

Why not +2?

Because it is cumulative with everything else. As shown above, alone it is quite weak, but it works together with magic weapons, expertise, anything and everything.

Exception

If this is the only item the player will ever have, I would put it at +3. Just make sure one character can't wear the other's too, so it should be a helmet or something else non-stackable.