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Tanks and fighters who stack physical defense shrug off ordinary damage. Penetration and defense reduction cut through it.
Defense reduces the damage you deal. Penetration ignores a part of it, and some effects lower the target's defense directly.
These items and this talent do one of those things, according to their text.
Numbers and effects below are the in-game text from our database, not typed in by hand.

Attack item ยท 2060g
Percentage physical penetration that grows with the target's physical defense.
Unique Attribute / Armor Buster. Unique Attribute: +30% Physical Penetration. Armor Buster: When attacking an enemy, gains 0.1% extra Physical Penetration for each point of the enemy's Physical Defense, capped at 30%.

Attack item ยท 2120g
Percentage physical penetration, plus extra attack range.
Armor Buster / Malefic Energy. Armor Buster: Increase Physical Penetration by 30%. Malefic Energy: Increases Basic Attack range by 12%. When hitting a target, gains 10% Movement Speed for 0.5 seconds.

Attack item ยท 2010g
Flat physical penetration.
Unique Attribute: +15 Physical Penetration
Unique Attribute / Retribution. Unique Attribute: +15 Physical Penetration. | Retribution: Dealing damage to an enemy hero or Creep for 5 times in a row and grants 50% extra Movement Speed that decays rapidly over 3s. This effect has 8s cooldown.

Attack item ยท 1950g
Flat physical penetration.
Unique Attribute: +15 Physical Penetration
Ambush. If no damage is taken or dealt within 5s, the next Basic Attack will deal extra Physical Damage equal to 160 plus 40% Physical Attack, and inflict 40% slow effects upon the target for 1.5s.

Attack item ยท 830g
Flat physical penetration.
Unique Attribute: +12 Physical Penetration
Unique Attribute. +12 Physical PEN

Defense item ยท 1910g
A defense item whose damage over time lowers the target's defense.
Burning Soul / Curse. Burning Soul: Deals Magic Damage equal to 1.2% of hero's Max HP per second to nearby enemies. This damage is increased by 125-350 against creeps and minions (scales with level). Curse: Dealing damage with Burning Soul will reduce the Target's Hybrid Defense by 3-9 for 3 seconds.
Assassin Emblem talent
An Assassin emblem talent that adds penetration, physical when your extra physical attack is higher than your extra magic power.
Gain 5 Adaptive Penetration. Gain Physical Penetration if extra Physical Attack is higher than extra Magic Power, otherwise gain Magic Penetration.
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