In the title. Tanks can be truly buffed, bruisers would not want this item. Now supports can take this (not ap supports like morg-annie-zyra-brand) This could help the damage problem we have.
good concept in theory, not in reality
If the numbers are too low, like 10%, the damage decrease isn't that much in comparison to how much less damage you take.
If the numbers are too high, tanks will not be taking any damage whatsoever, though still dealing damage because squisheis taking 70% damage is stil a lot.
imagine a 
with this item and his ult.
what about something like a reverse 
not as high in armor stats but , you heal slightly based on some physical damage taken ,... i dont play tank .. but , most tanks i see , get shredded so easily :/ with all these tank kill items and spells ,... thornmail is good , but if you have any type of life steal, never seemed like it did to much , from what iv seen in my games at least
It would have to not work on turrets or epic monsters. It would make tower diving sooo easy.
