By itself, Thornmail'sThorns reflects at least 16 magic damage on being hit.
Thornmail is a particularly effective defensive option when facing down opponents whose damage is dealt primarily through basic attacks, or against opponents that utilize healing from attacks to survive fights (such as life steal).
Unique Passive - Thorns: When struck by an attack, deal 10 (+ 10% bonus armor) magic damage to the attacker and inflict them with 40% Grievous Wounds for 2 seconds if they are a champion. Immobilizing enemy champions also applies 60% Grievous Wounds for 2 seconds.
New Effect: Reflected damage now deals (+25% bonus armor) bonus damage.
The armor ratio will be affected by flat armor reductions down to 0 armor - while percent armor reductions will be ignored for the purpose of calculating bonus damage.i.e. The Black Cleaver will not reduce the reflected damage by reducing your bonus armor but a level 5 Puncturing Taunt will reduce the reflected damage by 6.25(25% of 25).Negative armor values will be treated as 0. Armor penetration will not affect the ratio.