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The wielder will deal damage to the opponent equal to 1/8 of the opponent's maximum HP if this Pokemon is hit by a contact move. Stacks with Rocky Helmet. If the wielder is hit multiple times by the same contact move, for example by Tail Slap, then the wielder will deal damage on every hit. If the opponent faints due to Iron Barbs damage in the midst of a multi-hit contact move, the move will end. If the opponent uses any of Bug Bite, Circle Throw, Dragon Tail, Pluck, Rapid Spin, SmellingSalt, or Wake-Up Slap, and then immediately faints due to the damage from Iron Barbs, then the effect of that move will not take place. The damage dealt is not blocked by Substitute.
While not the best it could have asked for, Ferrothorn's only ability is quite effective in helping it be the defensive behemoth it is known as. As if it wasn't defensive enough already on the physical side, opponents become less willing to hit it with Fire Punch if they're losing health.
Found in Dream World