In Super Smash Bros. Melee, it is a chargeable overhead vertical slash, being useful for edgeguarding, platform tech chases, and extending aerial combos outside the reach of Marth's normal aerials. True to its name, it deals high shield damage, and will break any shield when fully charged. Despite being able to break any normal shield, however, it may be perfect shielded without any penalty to the receiver. Roy's Flare Blade is a clone of Shield Breaker, except it deals much higher damage and knockback and has an explosion effect, but charges slower and deals recoil damage at full charge.
In Super Smash Bros. Brawl, the move has had a change in animation: instead of an overhead vertical slash with a large arc area, it has become a single powerful stab. Marth also moves a small step forward when unleashing the move, which gives it some decent range. The style used now is much more in line with Marth's battle animations in the Fire Emblem games. Although Brawl's Shield Breaker charges and releases faster and is easier to sweetspot, it is weaker, laggier, and has less horizontal and vertical reach - a nerf caused by the animation change -thus removing much of the edgeguarding ability it had in Melee.
In Super Smash Bros. 4, the removal of shields no longer taking reduced damage from attacks makes it even easier for Shield Breaker to shatter shields, but is slightly offset by the move having less shield damage. Additionally, an uncharged aerial Shield Breaker now sends Marth forward a small distance, which increases his recovery potential. Despite these useful buffs, Shield Breaker in SSB4 remains slow enough for players to react to it, has no effect on powershields, and suffers from the universal nerf of Marth's sword range, making it much harder to land the move without hard reading shield habits due to the Falchion's drastically reduced range. This is further harmed by Marth not taking a step forward when he uses the move (regardless of whether it is fully charged), giving this move lower range than its previous incarnations along with higher risk.
The move was also given to Lucina. Her version of the move inflicts the same amount of damage regardless of whether the opponent is hit by the tip of her sword or not, much like most of her other sword attacks. She also has a different pose when charging and using the move, being highly exaggerated compared to Marth's charging stance.
In Super Smash Bros. Ultimate, the move can now be angled, allowing it to hit lower or higher targets. It additionally gained an unusual property where it deals 1.15× more damage if it hits an opponent's head. This is not dependent on the attack's charge level or whether it is tippered or not. However, if the attack hits a different hurtbox alongside the head hurtbox, it will not receive this damage bonus. Hitting an opponent's head also does not deal more knockback, despite the damage increase.
Reverse Shield Breaker
In Brawl, SSB4 and Ultimate, most neutral special moves can be reversed by tapping the control stick in the opposite direction a few frames after the neutral special move has been activated. Shield Breaker can be reversed with no increase to damage or knockback and no reverse animation; this makes reversing only useful if it is used to attack an enemy who has moved behind Marth. It is interesting to note that when Marth activates a reverse Shield Breaker during his running animation, he will turn around and slide forward slightly while charging or releasing the attack. This is probably because the running momentum has switched direction with Marth, since there is no animation for the reverse Shield Breaker and pushed him forward as if he did not turn around at all. This applies to Lucina as well.
In Brawl, Shield Breaker has stages of damage and knockback; the stages change when the attack is charged until it reaches the last stage, when the attack will commence. There are four stages in total with Shield Breaker - stages 1-3 are during the charging process. These stages always do the same damage even if the attack is tipped, which means tipping these stages will only increase knockback. Stage 4 is when the move is fully charged; this attack has multiple hitboxes which get stronger as they approach the tip of the sword. The stage the move is on can be determined by listening to the "clack" sounds Shield Breaker makes - stage two commences on the third "clack" and stage three can be performed by predicting the fifth "clack". Stage three is difficult to perform because there are only a few frames before the shield breaker is fully charged; attacking in these frames will let the player use this attack. Stage four is when the attack is fully charged and where it is strongest in both knockback and damage (unlike most chargeable moves, which have the penultimate stage as the strongest).
Special Move customization was added in Super Smash Bros. 4. These are the variations:
The move's ability to break shields is based on Marth's weapon of choice within the Fire Emblem series: the Rapier, a weapon unique to him in his games. The Rapier had the benefit of being able to gain a damage bonus when used on enemy cavalry or armored opponents, such as knights. In addition, when Marth wields the Rapier in Fire Emblem: Mystery of the Emblem, Marth has a unique animation that greatly resembles that of Shield Breaker, with him leaning back and stabbing the opponent. Marth has a similar animation when using any sword in Fire Emblem: Shadow Dragon and Fire Emblem: New Mystery of the Emblem. The Rapier is also exclusive to the Lord class in Awakening, and Lucina, Marth's clone, also has this weapon and starts as a Lord in Awakening.