Thornmail

A little insight on this item:

It's counterproductive against itself. It reduces the damage you take, and of that damage you only fire back 30% as magic damage. This is useless on mages who wish to survive against attack damage carries, as they would likely already have invested in magic resist.

Superior (most of the times) items are Frozen Heart or Randuin's Omen, or Zhonya's Hourglass if you are a mage. Why? Because they are better at preventing the phsyical damage taken through their slow, and can't be countered by pure lifesteal.

Though it's your choice to take Thornmail, it's only viable against a glass-cannon setup.

Edit: I was unaware that the damage reflected is before damage migation, making this item a lot stronger on pure tanks. Though for any champion that does not have the skillset/health availible, any of previous mentioned items might still be worthwhile. Thank you for the info.

Not really BBerger...
Thornmail is in fact really imba, and it's not counterproductive, but all the way arround.

It does return the 30% of the damage before the damage mitigation (true story) and that is what makes it (imo) too hard to counter for ADs (specially on Tanks) and really hard to counter with lifesteal.

An example:
I'm Tryndamere, and I hit someone with Thormail for a 1.000 Raw Crit.
Thormail returns then to me 30% of that damage (300 damage) as Magic damage.
Then, the guy wearing Thormail reduces (for example) 50% of physical damage with his armor value, so I'm really only hitting him for 500 damage, while he returns 300 damage to me.
If I had 25% of lifesteal (a full charged Bloodthirster for example), I would be healing 25% of the 500 efective damage, healing back 125 HP; still getting efectively hurt 175 for every 500 damage I'm doing to that guy.

The real problem starts with tanks (or Armor Stacking people).
If my 1.000 damage was reduced by 75% by a Thornmail wearer; I would be making 250 damage (healing only 62 with a big 25% lifestealing ammount) for every 1.000 Raw Crit, while getting hurted by 300 Magic Damage.
Meaning that it costs me 238 HP to hurt 250 HP to a tank that has more HP than I do, even taking Lifestealing into account.

However, it's truth that the Magic Damage reflected can be reduced using Magic Resist (not counted on the examples) or that you can make a lower % of your total damage being reflected with Armor Penetration; but Thornmail is not a usless as you seem to think; and it's in fact more powerfull the most Armor/HP you have (and the wearer Magic Penetration helps; reason why Mordekaiser with MagicPen Boots/Scepter and Thornmail is so hard, also his shield reflects damage as if he was getting hit normally).

So it's not counterproductive against itself. You return the same damage to the attacker (no matter what Armor you have; since returns the freaking Raw Damage); and then reduces the damage by increasing your armor.
Imo, this item should be changed to return 50% of the real damage dealt or something like that; if a good tank gets it, AD can only hurt him (without suiciding) using a high combo of Lifesteal, Magic Resist and Armor Penetration (Last Whisker). Too expensive to counter an item that also gives survibality via increasing armor.

If I stack 2 Thornmail will I return 60% of the raw damage?

Does this return damage from Vayne's Silver Bolts?

I only see this working on Rammus since it stacks with his Defensive Ball Curl