Lethality is a skill in the Fire Emblem series. It was introduced in Fire Emblem: The Blazing Sword. The skill can instantly kill the enemy regardless of their stats or items. It is a skill learned by Assassins.
The Lethality skill triggers on the chance of the critical hit to instantly kill the opposing unit. The skill does not work on certain character (particularly named characters). The skill relied on the straight critical rate in its first appearances, happening 50% of the time when hitting a critical hit, but the series transitioned into using the Skill stat instead with later game. Awakening lowered the chance further by dividing the skill by 4 and making it so that it does not guarantee the attack will hit. The guard from Pair Up and Guard Stance will neutralize the attack entirely while Miracle can let the unit live. The Counter/Countermagic skills can reflect the damage, but do not necessarily kill the opponent.
It is the base skill of Assassin in The Blazing Sword, The Sacred Stones, and Radiant Dawn.
In Path of Radiance, it is the occult skill of the Assassin class, requiring the Occult Scroll to learn.
In Awakening and Fates, it is learned at level 5 for Assassins and Master Ninjas respectively