Originally Posted by Ballzofire
How does it counter lifesteal? Lifesteal counters the thornmail passive. If your healing back the same amount of damage your taking from the passive. Its not really a passive id like to have. Minimal amounts of Magic resist essentially cancel the damage it does. Its a niche item for 1 champion. This needs reworked or deleted with the current game there is no reason for this item to exist. Also even if a tank got a thornmail. As an adc i will just laugh and melt his team while i ignore him. Why would a tank want an item that makes u not want to attack him??? Yet another reason this item needs removed.
Here's the thing you're mistaken on.
Thornmail's damage return is pre-mitigation, while Lifesteal is post-mitigation. This means that Armor reduces Lifesteal directly, but does not affect Thornmail's damage return (it IS affected by enemy MR)
So, if you have JUST Thornmail, your armor at level 18 is going to be 161-190, which is 61.7-65.5% damage reduction. You're taking less than 40% of their AD per hit if they have no Armor Penetration.
Meanwhile, base MR being 30-52.5, Thornmail is returning 19.7-23.1% of their AD to them.
Just based on Thornmail vs Lifesteal, the attacker needs 51-67% Lifesteal to NEGATE Thornmail. It takes a combination of Lifesteal, Magic Resist and Armor Penetration to make Thornmail useless.