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.