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Shield Breaker is Marth's neutral special move in the Super Smash Bros. series since Super Smash Bros. Melee, and Lucina's in Super Smash Bros. for Nintendo 3DS / Wii U, based upon the Rapier's ability to deal extra damage to armored enemies in the Fire Emblem series. By holding the special button, Marth will charge up an attack with his sword that deals high damage and, as the name states, is capable of breaking opponents' shields at full charge. In addition, unlike most charge attacks, Marth automatically unleashes the attack when fully charged, rather than saving the attack for later, and can also be automatically performed by releasing the special button at any point during charging.
In Super Smash Bros. Melee, the Shield Breaker has Marth holding his sword over his shoulders to charge and releasing it as a vertically arcing slash. In Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U, however, the attack was changed to a forward thrust, decreasing its range, though it is also faster to charge and execute; the move's power was also decreased for Super Smash Bros. Brawl but was increased again for Super Smash Bros. for Nintendo 3DS / Wii U. Lucina also uses the Super Smash Bros. for Nintendo 3DS / Wii U version of the attack. Also starting with Super Smash Bros. Brawl, Marth (and Lucina in Super Smash Bros. for Nintendo 3DS / Wii U) can turn around while charging by tilting the control stick in the opposite direction soon after starting to charge the attack, though unlike most moves this does not increase the move's damage output.
Also in Super Smash Bros. for Nintendo 3DS / Wii U, this move has two custom variants which are shared between Marth and Lucina. The first custom variant is Storm Thrust, deals less damage than the normal Shield Breaker but creates a gust of wind that blows opponents away, the size of the gust also depending on how much the attack is charged. The second custom variant is Dashing Assault, which like Storm Thrust does not deal as much damage as normal but Marth or Lucina charges forward when attacking, allowing them to strike multiple opponents more easily; the distance they move forward is also determined by how much the attack is charged.