A change to Black Cleaver to help with the Armor Penetration stacking issue.

Giving Black Cleaver % Armor Penetration instead of % Armor Reduction shouldn't change anything on its own. % Armor Penetration stacks multiplicatively with itself after all. This means that 100 Armor against 30% Armor Reduction and 45% Armor Penetration ends up being 38.5 effective Armor. Against 30% Armor Penetration and another 45% Armor Penetration it also ends up being 38.5 effective Armor, not 25. But by making it grant % Armor Penetration we can gate the effect for other allies (including minions and towers, but not monsters).

Black Cleaver is supposed to be a core item for Fighters to still deal damage to Tanks and each other. It's not really supposed to be an item that massively boosts the damage of an allied Marksman or Assassin. That was just a nice bonus. This nerf would make Black Cleaver just as effective on the people who are supposed to or do buy it, but it would reduce how useful it is for the whole team.

With this change, a level 13 Rammus with 374 Armor would have about 43 more effective Armor (71 --> 114) against an almost fully stacked Armor Penetration enemy who does not have Black Cleaver himself. If the enemy is someone like Miss Fortune who does have Black Cleaver, he at least doesn't have reduced Armor which makes his passive and W more effective.