This magic weapon appears to be a longsword, +1 to anyone who tries to identify it, and it confers that item’s property while on your person.

Curse. This sword is cursed, and becoming attuned to it extends the curse to you. While cursed, you take a -2 penalty to attack and damage rolls made with this weapon. Until the curse is discovered, the DM secretly applies this penalty, assuming you are adding the item’s bonus. Until the curse is broken with remove curse or similar magic, you are unwilling to part with the weapon, keeping it within reach at all times. In addition, you have disadvantage on attack rolls made with weapons other than this one.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Combat, Cursed, Versatile, Sap