hey guys, I was playing with my tryndamare today and I noticed a strange damage bonus every time I made an critical strike, so I sold every item to see which one was doing that, wasn't Phantom Dancer, Infinity Edge, The Bloodthirster nor Berserker's Greaves, they I bought every think and build the Blade of the ruined king again, and evey time I did an critical strike a bonus damage was made! No I am not trolling, and unfortunately I can't put images because I don't know how print screen in game , but I will say how works.
build blade of the Ruined king, you must have chance of critical damage to do that "hidden unique passive" works, when you do an critical strike an extra damage will pop from the side of your real damage, and the damage varies with foe's actual damage ( I don't know how much %) then it popped from my mind: wait, the item already give an 4% actual damage + this hidden passive, isn't it OP?
with you folks wan't to test it go ahead
oh forget to mention, the extra damage is in physical damage

no that's the passive of the blade of the ruined king working. it's showing you how much you hit then what is added on by the item.

"UNIQUE Passive: Basic attacks deal 4% of the target's current Health in bonus physical damage (max 120 vs. minions) and heals for half that amount on hit."

because tryndamere is a melee champion he will always be Autoattacking and will activate the passive of the blade of the tuined king because it's a basic attack not an abilitiy