Pets that deal physical damage will apply stacks, even if they don't apply on-hit effects or spell effects.
The very first instance of physical damage does not get the benefit of the armor reduction. The physical damage dealt is calculated first, and the debuff is applied after. This means the full benefit of the reduction is seen on the sixth hit, after all five stacks have been applied.
Last Whisper provides more raw percent-based armor piercing than the full five Black Cleaver stacks (35% to 25%, respectively), but The Black Cleaver's armor shred benefits an entire team, while Last Whisper's only augments its wielder's damage.
Armor reduction and penetration effects stack, allowing an attacking character to benefit simultaneously from Black Cleaver shred, penetration runes, other items such as Last Whisper, and champion abilities that circumvent armor.
The armor reduction passive stacks with itself, purchasing two The Black Cleavers will cause physical attacks to apply two stacks per hit 5 stacks cap still remains.
Kayle'sHoly Fervor reduces the armor of enemies hit by her autoattacks by 7.25% for 5 seconds per stack. This debuff stacks up to 5 times (Max. 36.25%).
Renekton'sSlice and Dice, when empowered by having more than 50 Fury, Dice will reduce the armor of all enemies hit by 36.25 / 40 / 43.75 / 47.5 / 51.25% for 4 seconds.
Trundle'sSubjugate reduces the armor of his target by 40% to 55% in the next 4 seconds (the effect does not apply any stacks). The armor reduction lasts for another 4 seconds after the drain completes.