Thornmail reads "Reflect 100% of damage mitigated from attacks"
"mitigation" is not a defined Word, so
expectation: 100% of damage mitigated from attacks is reflected
result: does not return damage on dodge / shen spell / kindred spell / kassadin shield / redemption shield / couldnt yet test with knights bonus
(all these things "mitigate" damage)
so I do believe it means "mitigated by armor" & I do also believe that it works as intended, but It doesnt do what it says,
whether the do or the say is the mistake I dont know
but it would be nice if things would have correct description & I wouldnt mind items be stronger on certain champs.
its the case for damage items also