Lethality in LoL is flat armor penetration : it reduces part of the enemy’s armor before damage is calculated, so your physical damage hits harder, especially against squishy targets. It scales with target level, which means it’s generally strongest in the early to mid game and is most useful on bursty AD champions like assassins.

How it works

  • More lethality = more of the target’s armor gets ignored.
  • Against low-armor champions, it boosts damage a lot.
  • Against tanks or champions stacking armor, it becomes less efficient than percentage armor penetration.

Who builds it

  • Assassins and combo-focused AD champs usually benefit most.
  • Champions that rely on auto-attacks or sustained DPS often prefer AD, crit, or attack speed instead.

Simple example

If you have lethality and hit a target with low armor, some of that armor is removed from the calculation, so your abilities can burst them down faster. At level 18, lethality converts to its full flat armor penetration value.

TL;DR: Lethality makes physical burst damage better by cutting enemy armor, and it’s best against low-armor, squishy champs.