Noroki (5e Equipment)
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Weapon (greatsword), uncommon
This magical greatsword normally takes years to master, but the sword is surprisingly light and easy to swing. You can use your choice of your Strength or Dexterity modifier for the attack and damage rolls with this weapon. You must use the same modifier for both rolls. Creatures who normally don't have proficiency with greatswords may choose to have proficiency with this weapon.
Curse. If you choose to have proficiency in this weapon, or when you first make an attack roll with this weapon, you lose proficiency in all other martial weapons. You do not notice this curse until you make an attack roll with another weapon.