One of the hardest aspects of LoL to deal with currently are carries - they can cut through your armor and dps you down extremely rapidly. There will also be some very, very hard hitting magic users in the future, when the spell crit item comes. This item is intended to help neutralize both those classes of champions, but with an extremely high cost - not only in gold, but in taking up an item slot for no other benefit than its passive. Note that all effects of its components are lost as part of the upgrade.

The Neutralizer (600 gold. Total cost: 2040 gold)
Components:
Chain Vest (700 gold)
Negatron Cloak (740 gold)

Effects:
Passive: Your champion is immune to all critical strikes.

Edit: Costs can of course be adjusted for balance, these numbers are just to provide an example as a starting point. It could also become -50% crit damage (And have some AR/MR) or other such effects, if complete neutralization is undesired. However, I think that would actually make it stronger because then it could just be bought as part of AR/MR stacking, and part of the goal of it losing the AR/MR is to make it useless for anything except crit neutralization due to how powerful that is -- a specialized item with a high cost for a high potential reward.

As someone who pretty much only plays as Mundo, I can tell you that this item would render him almost completely useless.

-crit damage is a good idea. Negating crits is not.

I like the idea of an item that either reduces critical hit damage (e.g., -75% crit damage bonus) or else an item that protects you from one critical hit every X seconds.

make it negate debuffs like, silence and fear but no stun and slow, or simply make it reduce crit damage recieved to 1.5x damage only

if im not mistaken, isnt crit calculated after armor? if not I think it should, cuz the lower the base number, the lower the crit damage by a lot, since its a multiplive number.

another idea is also make armor reduce crit damage as well, such as whatever the armor reduces in physical damage, 1/4 of that reduces crit damage as well