Damage is reflected when hit by a basic attack, regardless of whether the attack deals damage. If the attacker is blinded, or the wearer is protected by a shield or invulnerability, damage will still be reflected.
This means that the Grievous Wounds, applied by Thornmail, will be effective with the basic attack that triggers the effect.
Thornmail is a particularly effective defensive option when facing down opponents whose damage is dealt primarily through basic attacks, or against opponents that utilize on-hit15px healing 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.