Let's give tanks an item to protect squishies from assassins

WILL OF THE PROTECTOR +60 Armor +45 Magic Resist

UNIQUE Passive: Whenever a nearby ally takes damage that would reduce them below 25% of their maximum health, they gain immunity to damage from the enemy that dealt this damage for 4 seconds, but 50% of the damage they'd be taking is dealt to you instead. (120 second cooldown)

Build Path: Chain Vest + Negatron Cloak + 980 gold = 2500 Gold

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So, with assassins deadlier than ever, adc's are turning to Sterak's Gage for protection. But is this the best option? Fighters certainly should be able to stand their ground versus assassins if they build tankyish, and tanks, while not able to kill the assassin by any means, should have a chance to survive. But marksmen and mages are countered by assassins - if they want to live, the best option should be to find a tank and stick to them like glue. Problem is, tanks can't deal with assassins very well either right now - by the time they notice the assassin, their little buddy is dead. So what's the solution?

Let tanks do their job - protecting their more vulnerable allies.

Now, I've tried to gear this item versus assassins without making it must-buy for every tank (although it may become must-buy if there's always assassins to worry about, but that's a different problem). Note that your ally is only immune to damage from the enemy that triggered the ability. Long story short, if they get targeted by more than one person, they're still dead - this item can't help them. The goal here isn't to make assassins useless - it's to make them think. If they rush in, knock someone down to 25% to trigger the passive, then rush out via hypermobility, their mage/adc can then finish the job. The point is that they can't do it on their own.

Also, if the assassin is a persistent lil' bugger, you'll wind up taking damage as well. This is intended to keep anyone short of a dedicated tank from buying the item. Another reason it's attractive to tanks is that it makes enemies want to target you - if you're dead, the item won't work, so the assassin can wreck house as he/she pleases. Tanks love taking damage if it means someone else isn't.

Now I don't claim this is a perfect solution - maybe 100% damage reduction is too anti-fun for assassins, maybe the item is too cheap/expensive, maybe the stats suck way too much (they're worse than Guardian Angel's), or whatever. I ain't horribly knowledgeable about League balance. Thus, I bring the proposal to the Boards for review and criticism. Whatever you do, don't leave without saying something.