Weapon (longsword), legendary (requires attunement)
Aamor Fin is a beautiful, ancient, lyrium-laced longsword of unknown providence. Her silver flame shines in darkness, but cannot ignite flammable objects and gives off no heat. Sometimes, very softly, she sings.
Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Sap. If you hit a creature with this weapon, that creature has Disadvantage on its next attack roll before the start of your next turn.
Notes: Bonus: Magic, Damage: Slashing, Damage: Slashing, Damage, Combat, Versatile, Sap
If you're proficient with a longsword, you already add your proficiency to the attack roll. Did you mean add your proficiency to the Damage roll, or that the magical bonus is equal to you proficiency bonus? For example, a +2 proficiency makes it a +2 longsword, up to the maximum of +6 proficiency making it a +6 longsword.