Marth and Lucina's versions of Critical Hit sees them rushing forward to deliver an immensely powerful slash that deals a devastating amount of knockback; in every game it appears in, it is an unavoidable one-hit KO if directly in the line of sight of a blast line, even on the largest possible stage of each game.
Roy's version of Critical Hit involves him staying still and spinning the Binding Blade in a slow circle, before unleashing a fiery explosion in front of him. Unlike Marth and Lucina's Critical Hits, Roy's version is not a one-hit KO, though it is capable of comboing from some of his moves, including his throws.
For both versions of the move, a HP gauge from Fire Emblem is displayed alongside each hit character if the move hits, which starts at full and quickly drops to zero.
Critical hits are a staple feature of many role-playing video games (including other series seen in Smash, such as EarthBound, Final Fantasy, Xenoblade, and Dragon Quest). They are generally rare attacks that have a certain chance of occurrence, with the payoff usually being an increase to the amount of damage dealt, e.g. doubling the current amount. In Fire Emblem, critical hits inflict three times the damage that a normal strike would (with the exception of Genealogy of the Holy War and Thracia 776), and the odds of executing them are based on the unit's skill stat and on the properties of the weapon they wield. The Critical Hit Final Smashes reflect the source material's extreme power—often far more than enough to kill an enemy in a single hit—with their high damage output and, in particular, the extremely high knockback of Marth and Lucina's shared version.
Executing a critical hit in Fire Emblem is almost always accompanied with a distinctive, flashy attacking animation, and these animations form the basis of each of the two Critical Hit Final Smashes. Marth and Lucina's shared version loosely resembles Marth's animation when performing a critical hit with Falchion, Mercurius or a rapier in Fire Emblem: Shadow Dragon and the Blade of Light. Roy's Critical Hit is a near-exact match for his attacking animation while wielding the Binding Blade in Fire Emblem: The Binding Blade, right down to the airborne version's recovery flip, performed in all cases in his game of origin when Roy sheaths the sword after striking; this animation is identical for all attacks he makes with the Binding Blade, although it only catches on fire, as it does in Smash, for critical hits and long-ranged attacks.
The gauges that appear with Critical Hit are a nod to the hit point health system used in Fire Emblem, and are designed to resemble the HP gauges used in specific games. In Brawl, Marth's gauge resembles that of Fire Emblem: Mystery of the Emblem, and contains 52 units of HP, matching the maximum possible amount of HP in Shadow Dragon and the Blade of Light and Mystery of the Emblem (though their remakes increased this to 60). From Smash 4 onward, the gauge used for all three characters is patterned after that of Fire Emblem Awakening and counts 80 units of HP, again matching the maximum possible HP in that game under ordinary circumstances.
In more recent games, units announce their critical hits (and special skills such as Aether) with a bright flash and by uttering a dramatic quotation. Lucina's utterance of "Time to change fate!" in her Final Smash is taken directly from this: in Awakening and subsequent games, the Japanese version of the line, 運命を変えます！ ("I will change fate!") is one of her critical hit/special skill lines. The line is usually translated "I challenge my fate!" in English Fire Emblem localizations, but not in Smash.
Marth and Lucina
The user raises their sword in the air, where it catches the light and shimmers brightly, before dashing forward with blazing speed to catch any unfortunate opponents with an extremely powerful slash. In all installments, Marth does not say anything while performing this move (apart from a simple grunt).
In terms of knockback, it is by far the strongest Final Smash in Brawl, SSB4 and Ultimate, and its power and damage also make it one of the strongest attacks in all of the games that it appears in. It deals 62% damage in Brawl (60% base plus 2% fresh) and deals more than enough knockback to KO any character at 0% under most normal circumstances. However, the power of this move is somewhat hampered by its larger time frame in which to perform a dodge (the time during which the user raises their sword), making it deceptively predictable and avoidable. Landing the blow is hence easiest against opponents suffering from ending lag or helplessness, or who are simply unaware, as all other players in range can completely dodge the move by air dodging, sidestepping, or simply moving away with good timing. In Brawl, Marth can also get a guaranteed Critical Hit if he forces his opponent into an aerial grab release, although this only works on some characters. In SSB4 and Ultimate, the startup slows down time by a slight amount, making the main attack harder to dodge.
Until the attack animation completely ends, the entire sword carries the move's hitbox, meaning that the attack can also hit anyone directly behind the user when they raise their sword. Because the attack itself gives such an high amount of freeze frames, it is also possible for a fighter to walk into the user's sword once it hits another opponent and still be given a OHKO, even after half a second. Critical Hit only hits once, so with proper timing, moves that provide armor can be used to survive it. Of course, this will not stop the target from taking the move's full damage.
On the ground, the initial dash will follow the platform's contours, and the move will end without the user swinging if they reach the edge. In the air, the user will travel at high speed directly forward. The dash can be canceled immediately by pressing an attack button, which also causes the user to swing their sword. The dash will automatically travel its maximum distance if uninterrupted, which is approximately the same distance as the width of the largest area available in Brawl's Stage Builder; missing with this attack on the ground on a walk-off stage, or in the air without any intervening terrain, usually results in a self-destruct unless the player manually cancels the user's dash. The distance the Final Smash grants allows it to be used as an extremely effective horizontal recovery move if needed, although this is situational.
In Brawl, this attack has such immense knockback that it is possible for the main victim to KO other opponents by simply knocking into them.
In Super Smash Bros. 4, Critical Hit can ignore all forms of armor. It was also given to Marth's clone Lucina, with identical functionality. When she performs the move, Lucina announces "Time to change fate!" (運命を変えます！, I will change fate!) instead of merely grunting.
Roy's Critical Hit involves him standing in place while spinning the Binding Blade in a backwards arc around him with two hands. It then flashes with light (similar to the startup of Marth/Lucina's shared Critical Hit) before Roy swings his sword violently downwards, creating a massive fiery explosion in front of him. Roy will then sheathe his sword (a purely aesthetic animation that ends in a second); if he finishes the Final Smash in the air, he leaps a small distance upward, which does not count as a second jump. Similar to Marth's/Lucina's, a purely aesthetic health bar appears for every victim sent flying, showing them dropping from full health to zero. However, Roy's variant of Critical Hit is not able to pierce through super armor.
Unlike Marth/Lucina's shared versions of Critical Hit (which can KO at any percentage under normal circumstances), Roy's version only deals 40% and can only begin KOing at around 40%. It also requires careful positioning since Roy stands still, and the hitbox does not cover below Roy, making it possible to miss if the opponent is not directly in front of him. However, touching the Binding Blade during the move's startup (when Roy slowly circles his sword) traps victims within the swing, which leads directly into the final hit. This gives the move combo utility at low percents, as a forward/down throw or neutral attack can lead directly into a Critical Hit. As the swing starts from behind, it is easier to catch opponents behind Roy when compared to the front.
Overall, Roy's Critical Hit is more reliable at connecting up close and can be used as a niche combo finisher, though it possesses significantly lower range and lacks the extreme power of Marth/Lucina's respective Final Smashes.