A deeply embedded character weakness so fundamental to who they are that it ultimately causes their destruction or downfall.
A fatal flaw is a character flaw elevated to its most extreme consequence - it doesn't just cause problems, it causes ruin. Rooted in the Greek concept of hamartia (a tragic error or misjudgment), a fatal flaw is typically inseparable from the character's identity. It's often the dark side of their greatest strength: a brilliant mind becomes arrogance, fierce loyalty becomes blind devotion, deep passion becomes obsession. What makes it fatal isn't the flaw itself but the character's inability or refusal to recognize and overcome it before it's too late.
Fatal flaws give stories a sense of tragic inevitability that readers find deeply satisfying, even when it breaks their hearts. When you craft a fatal flaw well, readers can see the disaster coming and still can't look away. It transforms a character's story from a sequence of events into a meaningful examination of human nature and the price we pay for being who we are.
Hamlet's indecisiveness and overthinking - his need to be absolutely certain before acting - delays his revenge until the body count includes nearly everyone he loves.
Gatsby's fatal flaw is his inability to accept that the past cannot be recreated, pursuing an idealized version of Daisy until it literally gets him killed.
Captain Ahab's monomaniacal obsession with the white whale is the purest fatal flaw in American literature - magnificent, terrifying, and utterly self-destructive.
Bojack's self-destructive need for external validation and his inability to accept love he hasn't 'earned' systematically ruins every good thing in his life.
The best fatal flaws are virtues pushed past their breaking point. Tie the flaw to something the character is rightly proud of - that's what makes it truly tragic.
Seed the fatal flaw throughout the story. The reader should be able to look back and see it operating in every major decision the character made.
A fatal flaw should be woven so deeply into the character's identity that overcoming it would require them to become a fundamentally different person. That's what makes it fatal.
A fatal flaw is internal and psychological, not situational. Being in the wrong place at the wrong time isn't a fatal flaw. Being constitutionally incapable of walking away from a fight is.
Identify your character's greatest strength - the quality that makes them who they are. Now write a scene where that exact quality, pushed to its extreme, causes irreversible damage to something they care about. The character should not realize that their best quality caused the harm. That blindness is what makes a flaw fatal.