Like the title, but lets say the party is about to fight a pirate captain and he wants to attempt to frighten them in a way where they will gain the frightened condition. How should i roll play that to make them actually frightened and not just tell them that they are scared?

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Solution in case anyone wondered about this too (MAY BE NSFW OR TRIGGERING TO SOME): i think what i will do is have some non interaction build-up where the players hear about this crazed pirate lord and about his merciless and savage killings. Upon finding the Pirate maybe the players will see him brutally murder someone and i will explain very deeply how this skeletal (in the moonlight) pirate punctures through a human chest and rips out a heart. Or brutaly uses the stock of a gun to beat the person to death, and then subtly have the players roll a high DC wisdom save.