Why isn't there an item that returns magic damage like
returns physical? It will go perfect with my idea for a new champion! :D http://boards.na.leagueoflegends.com/en/c/skin-champion-concepts/P1BXnUW9-vargh-the-absorber
Because of a lot of reasons that I and several others have written and rewritten multiple times. There are code issues, balance issues, the fact that there is no uncontrollable basic attack, but most of the spells are AoE, skill shots, bounce spells and/or uncontrollable in some other way...
However, I always find it best to answer this question with a question.
How would you deal with a champion that bought regular Thornmail with this?
Primarily because physical damage is sustained (and often complemented with lifesteal), while magical damage tends to be bursty.
If you're a marksman and attack someone with Thornmail a couple times, you can be like "ow, I'm taking damage, maybe I should disengage", whereas with the AP version someone like LB or Veigar would unload their combo on someone and delete themselves unless they had also built a lot of MR and/or SV.
There is similar reasoning as to why SV is not as available as LS; burst damage that also comes with massive self-heal is very hard to balance. As a non-League example, see Warlocks in WoW PvP -- Death Coil (which is a long-cooldown instant nuke with large self-heal which also applies a short hard CC) makes it VERY VERY difficult to duel a warlock with it up unless you're something like a Rogue that can stunlock/burst them down. Even if you catch them off-guard and take off a big chunk of their health, they push one button and seriously hurt you and heal themselves, then start laying on DOTs.
First off, may not be the best example of a fun or healthy item in the first place, but it's not totally game-breaking because it only reflects autoattack damage, which is highly controllable. Magic damage, on the other hand, mostly comes from spells, many of which are AoE and most of which aren't as reliable as point-and-click autoattacks. A team full of players with this hypothetical magic Thornmail could cause any mage to one-shot themselves upon landing a successful spell, which you clearly wouldn't want to have, and just one of those items could accidentally give mages a really hard time even if they were aiming for another target.
More or less for the same reason why there isn't an AD Zhonya's. It screws up the balance far too much.
Thornmail is a direct counter to ADCs because they put out constant DPS, however it is not effective at returning damage to AD Bruisers whose damage comes from their AD scaling spells, such as Riven or Renekton. If you put in an AP Thornmail then you're simply countering every single AP on the enemy team. Also, only a select few AP champs build any form of spell vamp, whilst ADCs have the option of building life steal almost every game which directly counters Thornmail.
In the current game, the AP Thornmail is Banshee's Veil. It is a CD based nullification of a single spell, when played properly this is usually somebody's engage spell, like Charm or Malphite's Ult.
watch teemo plant a shroom
stand next to it, even if it takes 7 minutes, and wait for teemo to be low hp
walk over it
