Does a magic weapon require the attuner to be proficient?

Question

Similar to this question, but this refers more toward proficiency rather than the word "wield".

The party has recently captured a greatsword from a defeated enemy. It has the property of conferring disadvantage to the saving throws of unfriendlies within 10 feet when drawn. It's a homebrew item, so the precise wording is not vital. I am comfortable to reword the ability to be more in line with existing items. It was used by a fallen paladin and was intended to convey the opposite feel to a paladin's normal aura of protection.

Here is the meat of my question. The wizard in the party wants to use it to confer saving throw disadvantage upon enemies. I'm not terribly concerned about how powerful this might make him; I have an unlimited supply of enemies. But it is a heavy, 2 handed weapon, with which he is not proficient (unless he wishes to work toward a feat or multiclass to confer proficiency).

Question: In order to use the properties of a magical weapon that requires attunement, does a character need to be proficient in said weapon?

Like I said, this is a homebrew, so I'm not terribly concerned about this particular item. I'm looking for a general principle, extrapolated from existing rules and similar items.

Answer 1

There's no implied requirement for any given user to be proficient with an item in order to attune to it

Borrowing from this answer to a similar (but unspecific) question, the key take-away I've made is that while Magic Items are fully allowed to specify restrictions on who can attune + use them (which can themselves require the user to be proficient), if it doesn't say it requires proficiency, then it doesn't.

I think this stands to reason given the prevalence of magic items that don't have proficiency doled out under normal circumstances. Items like bracers, cloaks, and wands—the latter of which I think is the most interesting in this context—don't offer the ability for wielders to be proficient (for, arguably, obvious reasons), yet so long as the user fulfils any prerequisites the item does demand, they'll be able to use it.

Caveat: How does the effect work?

If an item requires the user to do something, either for attunement or use of the attuned item, where that something represents something that cannot be done without proficiency in the item, then that implies a requirement of proficiency. Considering the following item I've made up on the spot:

Magical Platemail of Effervescence

(Requires Attunement) Whenever the attuned casts a spell while wearing this armor, all affected targets begin to emit bubbles from their nostrils

A character cannot cast spells while wearing heavy armor unless they have proficiency in heavy armor. Therefore, proficiency in heavy armor might not be a directly stated requirement of attunement for this item, but the user would be unable to invoke its effect without being proficient in heavy armor.

In your specific case, the Wizard can probably use the sword just fine

What the wizard needs to do in order to invoke its effect is merely draw the sword—an action which in-and-of-itself does not require proficiency. Proficiency is only required to make the wizard competent at attacking with the sword, not to merely hold it or draw it from a sheathe.

You didn't specify conditions beyond that, so there could be tricky parts to the sequencing: does the effect go away if they put the sword away? Do they have a hand free to perform the somatic/material components of the spell they want to gain the effect on? But as long as those considerations are met, then the wizard would be perfectly capable of gaining the benefit of the sword, even without proficiency.

Answer 2

There's nothing inherently in the requirements for attunement that say that one needs to be proficient in the item in order to be able to attune to it. And the only thing that proficiency with a weapon does is allow one to add one's proficiency bonus to its attacks. So there's no general requirement, and unless your item specifically says otherwise, your wizard could attune to the sword and use its special abilities, just like any other magical item. It's no different than not needing a special proficiency in order to attune to a ring or a wand, really.

In a really quick search of some published magic items, all it says in their description (such as the Dancing Sword) about proficiency is that one with proficiency gets to add one's proficiency bonus to the attack roll. Presumably if one couldn't use the item at all without that proficiency, it would say so.

One could certainly design your homebrew magical item to have an attunement prerequisite of having proficiency with greatswords, and I wouldn't think it'd be out of place or bizarre in any way to do so. The more common way to do it would be to have a prerequisite of being one of the martial classes, I think, but it's your item and your group and you can handle it however you want.

Answer 3

Proficiency does not impact using the "Magic" of a magic item

The proficiency with the weapon simply means that attack rolls with the weapon includes your proficiency bonus.

Unless the attunement prerequisites include proficiency with a weapon, which no existing magical weapons do as far as I am aware (although yours might because it is a homebrew item), then the magical properties would not be affected by whether the one attuned to it has proficiency with that weapon type or not.