There are many stats in League of Legends, including attack damage, ability power, ability haste, and more. Every champion has their own set of stats they want to prioritize for maximum efficiency, and one of the core stats for the assassin archetype is lethality.
This stat was introduced during the 2017 preseason to replace flat armor penetration, which became a problem with items that allowed champions to easily snowball out of control. Instead of ignoring the armor which the flat armor penetration gave before, lethality is a scaling effect that depends on the player’s level.
The formula for calculating how much armor you’re going to ignore is: flat armor penetration = lethality × (0.6 + 0.4 × level ÷ 18).
As a result, at level one you’re going to always have at least 60 percent of the flat armor penetration the item gives you. As you level up, you’re going to be receiving a portion of the second equation of 0.4 multiplied by your level and divided by 18. This stat stacks additively and cannot reduce a target’s armor below zero.
Lethality is the sister stat of the bonus armor penetration and is applied to the target after taking out the bonus armor penetration. In a scenario where you’d have 20 flat armor penetration from lethality and 30 bonus armor penetration, the target would be losing 30 of their armor, before losing the 20 flat amount. With 200 armor, for example, the target would be reduced to 140 by the bonus armor penetration and then to 120 by the flat armor penetration.
The stat is useful for assassin champions who want to snowball, since it will increase their damage early on by a nice margin, while also scaling throughout the game the more levels you gain. Some AD carry champions like Senna, Draven, Jhin, and Varus can benefit of this stat quite a lot if they can get ahead of lane. Due to most bottom lane champions having a small amount of base armor, you can punish them quite hard and ensure you get a good transition into the mid game.
While lethality is a great stat and can help you snowball games, it is not that efficient against champions who build a lot of armor. In such scenarios, going for lethality items might not be the best choice. Going for bonus armor penetration will be much more efficient since you’ll ignore more armor compared to lethality.