fatal

/ˈfeɪtəl/
adjective
  1. Causing death or ending in death.
    • Doctors warned that the disease could be fatal if not treated.
    • The driver suffered a fatal injury in the crash.
    • A fatal mistake by the pilot led to the plane's destruction.
  2. Causing disaster or complete failure; very harmful.
    • A fatal flaw in the plan was that no one had checked the weather.
    • One fatal error in the code caused the entire system to crash.
    • The team's lack of preparation proved fatal to their chances of winning.