Tank Magic Resist Itemization
Magic resistance itemization for tanks is uninteresting due to the nature of mage damage patterns and catch all stats on spirit visage
As it stands, you can build very specifically to deal with AD champions.
: Deals with champions like Xinzhao who rely on attack speed. Helps chamions who need their abilities a lot.
: Reduces damage from many adcs and champions like Yasuo
: Another item that punishes attack speed. Can be built against high AD teams to reduce damage by a lot.
: Helps immobile champions engage fights. Not always the best choice against certain types of damage.
: Improves the dueling ability of champions by increasing ability frequency and cc
: General purpose armor item for tanks that need to stick to targets. Not always the best choice against certain types of damage.
For Magic resistance though, there isn't much:
: General purpose magic resistance item. Deals with damage from most any magic source.
: It doesn't even have magic resistance, but after building a spirit visage this is your next best item to help survive mages.
/
: They aren't specifically magic resist items, but once you have warmog's armor they are your next best items to help survive mages.
I propose that there ought to be at least 3 unique magic resistance items, each with a specific purpose. Each would individually be slightly weaker than spirit visage, having less hp and magic resistance for example, but would have effects catered to reducing the damage of certain types of mages.
For the common mage/burst mage, an item that grants a shield upon taking 15% or more of your HP in magic damage with a 10 second cool down.
Example Champions:

For the DOT champions, an item that drastically increases hp regen for 3 seconds each time you take magic damage.
Example Champions:

For ability spam champions, an item that gains a stack every time an enemy ability is cast near you. At 3 stacks your next auto attack heals you for a % of your max hp.
Example Champions:

is so bad you even forgot to mention it.
but with mr
on steroids to deal with ap dps
