What if there was a 3000 gold tank item that reduced a flat amount of damage

not just CoC but Fearless is really really nice when you engage and then if you are going CoC taking Runic ARMOR

I think OP meant reduction against individual attacks. Sort of like Tough Skin mastery, or Doran's shield.

Steadfast Bulwark 500 HP 150% base health regeneration Passive - Unwavering. Blocks 5 (+10% of armor against physical, +12% against magical damage) from enemy champion attacks, applied after resistances. 25% effectiveness against damage over time.

So if you had 100 armor, and an enemy with 100 AD dealt a basic attack, you would recieve 50 damage without Steadfast Bulwark. With this item, it would reduce it by an additional 5 (+10) damage, reducing it to 35 damage from 50. These are just general percentages, I'm no balancing expert.

Better change it to scale with bonus armor/MR, so only tanks would build it.