Oh man, I love it when i play toplane and I get pelted/permaCC'ed by some champion like Ryze or Lulu with no way of fighting back. But hurr durrr such an item will make ap casters useless against tanks! Please, if your AP caster is trying to focus down a tank in a teamfight you deserve to lose.
YumaS2Astral5 votesCan we have an armor version of 
? I love when i play ADC and I have to purchase a MR item to counter 
Not every AD/armor item needs to have an AP/MR counterpart.
JustMyBassCannon2 votesNot every AD/armor item needs to have an AP/MR counterpart.
That and you have Zhonya's for AP instead of Quicksilver.
JustMyBassCannon4 votes[{quoted}](name=Yeezus the God,realm=NA,application-id=3ErqAdtq,discussion-id=LQBjARtN,comment-id=,timestamp=2015-06-08T22:28:47.838+0000)
But hurr durrr such an item will make ap casters useless against tanks!
That's not the problem with a spell Thornmail.
First problem. Spells are not restricted to a specific damage type. Pantheon's kit is physical damage spells; if you don't believe me, just drum up a vs AI game and buy Liandry's Torment. Thornmail is a reversed on-hit effect, thus a spell Thornmail would be a reversed spell-hit effect (inverse to Rylai's, Liandry's, etc.), meaning that it would reflect any kind of spell's damage.
Second problem. Someone buys both Thornmails. (a) They now have high resistance and reflect any damage thrown at them. (b) Restricting them to one or the other doesn't mean anything, because that means they'll be able to change it somehow if they want.
Third problem. Because it's a reverse spell-hit effect, it's treated as spell damage. This means that, if two people pick up spell Thornmail, the damage numbers can easily rebound until they hit the thousandths place in digits. Riot would need excessive safeguard code to make sure it doesn't do this, otherwise it would cause systemic lag and crashes from information overload.
Fourth problem. Thornmail exists to counter Lifesteal. Spell Vamp doesn't need a counter item.
NapaIm3 votesBecause an assassin would literally kill themselves after a rotation, and as an adc player I would buy this for that sole reason.
Yimmortal2 votesby your logic then adcs all around should be buying thornmail because assassins like yi or fiora would kill themselves after one rotation am i right?
Z Statistic1 voteswhat are you talking about? This is EXACTLY what you do when you are looking for a defensive item and yi or fiora are on your team.
EffectFX2 votesNo.
PiVoRx1 votescan i have ad version of zhonya?
3tyson1 votescan we have an ap version of 
cus not only tanks siege
and if your a tank trying to siege you deserve to hurr durr
Beddlam1 votesNo, you can't.