How does the new Lethality system work? I have tried reading the patch notes but I don't seem to understand. Can someone simplify it for me?
Looking at the League of Legend's wikia, you have the following explanation:
Lethality is a new statistic introduced in Season 2017. It grants flat armor penetration using the following formula:
Flat Armor Penetration = LETHALITY × (0.6 + 0.4 × Target's level ÷ 18)
Basically, it works the same as armor penetration, but adding the level of the target as a parameter: the higher the level, the greatest the armor penetration.
There is also an example to understand how the damages are calculated:
Given 30 flat armor reduction and 30% armor reduction, and the target is affected by 10 flat armor penetration and 45% bonus armor penetration,
Target A has 300 armor (100 base and 200 bonus armor).
The 300 is reduced to 270 (90 base and 180 bonus armor) by the 30 armor reduction.
The 270 is reduced to 189 (63 base and 126 bonus armor) by the 30% armor reduction.
The 189 is considered to be 132.3 (63 base and 69.3 bonus armor) by the 45% bonus armor penetration.
The 132.3 is considered to be 122.3 by the 10 armor penetration.
Target A takes damage as if it has 122.3 armor.
Target B has 18 armor.
The 18 is reduced to −2 by the 20 armor reduction.
The −2 is not affected by any further calculations because it is less than 0.
Target B takes damage as if it has −2 armor.