"I am the one weakness in an otherwise invincible hero. I am the target an enemy will always seek. My story is 3,000 years old but my name is still used today. What am I?"

Answer: Achilles' Heel. In Greek mythology Achilles was invulnerable except for his heel, which his mother held while dipping him in the River Styx. He was killed when an arrow struck his heel. Today 'Achilles heel' means a fatal weakness in an otherwise strong person or system.