The death of the Son of God as a sacrifice for sin weakened the devil's power. It made the devil's power over death ineffective (1 Corinthians 15:56).
Philo, a first-century Jewish philosopher, wrote that "nothing enslaves the mind like fearing death" (Philo, That Every Good Person Is Free 3.22). Through his sacrificial death, Christ freed those who were slaves to the fear of dying.