DM: Ok, Warduke swings his mighty mace and crits Knuckles the Thief. Roll me a Wisdom Save.
Player: Ok. Uh, I rolled a 12?
DM: Not good enough. You saw your friend drop, so you drop your weapon and run your full movement away from him...
Player 1: Waitaminute! I'm a 7th level fighter! I've killed dozens of creatures, some of them I've critted. I'm a tough-as-nails warrior, I'm a veteran of the King's War according to my background, and my flaw is I never back down from a fight! How does it make sense I would lose my nerve and run like a green recruit just because I saw someone critted?
DM: Sorry, Warduke has the Frightening Crit ability. You failed your save, lost nerve, and are frightened and must run for 1 minute. However, you can attempt to make a save each round...
Player 2: (laughing): Brave Sir Robin ran away. Bravely ran away away..,