From the Super Mario Wiki
Piercing Blow is a badge in Paper Mario: The Thousand-Year Door. It can be bought from the Lovely Howz of Badges for seventy-five coins, and is rarely dropped after battle by Spiky Parabuzzies. If Mario equips this badge, he gains access to a new hammer ability called Piercing Blow, which can be used at the cost of two FP. When it is used, it acts the same as a standard hammer attack, but it completely ignores the enemy's defense when attacking them. The damage dealt to them is the same as if they had zero defense (if they already had zero defense, the damage is the same as what a regular hammer attack would have dealt). Unlike other badges where multiple copies of them can be obtained, equipping multiple copies of the badge has no effect whatsoever. Piercing Blow functions the same as D-Down Pound in Paper Mario. The Iron Clefts are immune to this and all other defense-piercing items and attacks except for Mini-Yoshi's Gulp.
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