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Magic defense soaks up mages. Magic penetration and defense reduction are the ways through.
Magic defense reduces magic damage. Magic penetration ignores a part of it, and some effects lower the target's magic 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.

Magic item ยท 1970g
Percentage magic penetration, with extra against targets who have high magic defense.
Unique Attribute: +40% Magic Penetration
Spellbreaker. When attacking an enemy, gains 0.1% extra Magic Penetration for each point of the enemy's Magic Defense, capped at 20%.

Magic item ยท 2000g
Magic penetration, and its hits lower the target's magic defense.
Unique Attribute: +10 Magic Penetration
Unique Attribute / Magic. Unique Attribute: +10 Magic PEN. | Magic: Dealing damage to enemy heroes will reduce their Magic Defense by 3+(0.3*Hero Level) for 2 seconds. Stacks up to 3 times.


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, magic when your extra magic power is higher than your extra physical attack.
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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