Each weapon in the game has an ability you can use while said weapon is equipped. Under certain conditions (usually by using the ability a weapon grants you) you’ll gain proficiency with that weapon, and if you max out your proficiency with a weapon, the character will permanently learn that weapon’s ability, being able to use it even without the weapon equipped. For example, the Iron Blade imparts the “Triple Slash” ability, and you’ll gain proficiency by using “Triple Slash”. Learn any one weapon ability to earn this trophy. Check out Weapons Expert for more details.