The user yells "PK... STARSTORM!", calling forth a rain of meteors ("shooting stars") which pass through the stage striking any opponents in their way.
PK Starstorm (localized as PSI Starstorm in EarthBound) first appeared in EarthBound and later appeared in Mother 3. In both games, the move involves using the character's psychic power to send down stars on enemies. However, in both games, neither Ness nor Lucas learn the move; instead the move is taught to their friends Poo and Kumatora respectively in story events. Brawl explains the inconsistency in both Ness' and Lucas' PK Starstorm trophies, saying that they were taught the moves by their friends specifically for Smash. Super Smash Bros. Ultimate offers a new explanation by having Ness and Lucas summon Poo and Kumatora respectively during the animation.
In the description for the move in EarthBound, it states that the move uses a method of "shaking off the stars". In EarthBound, PK Starstorm can deal between 270 and 900 damage, and is available in both "α" and "Ω" tiers of increasing strength (the latter only becoming available during the last stretch of the game). In Mother 3, the move can deal between 650 and 680 damage and only comes in a single, unlabeled tier. Also, in both games, the move is impossible for any target to dodge, but it can be reflected by sufficient PSI shields; this behavior does not translate into Smash.
The move differs slightly between the two characters who use it. Lucas' meteorites are yellow, angle slightly inwards towards the centre of the stage, accelerate as they fall and deal 14% damage each, knocking the opponent sideways. Ness' shooting stars are blue, are angled more strongly and outwards rather than inwards, and deal 20% damage each, knocking the opponent upward. In addition, Lucas spawns a larger number of meteorites than Ness, but Ness's are slightly larger.
These differing properties, particularly the meteorites' quantity, make Lucas's damage output more consistent, but make Ness's more volatile and thus grant him higher KO potential. Due to the meteors' randomness, there can be times where they can be easily dodged or even avoided without having to move at all. Getting hit by even one meteor, though, is likely to cause the victim to be hit by many others, causing heavy damage and a likely KO.
PK Starstorm received notable changes, which apply to both of its users: it no longer covers the whole stage, but consists of a fast stream of meteors which covers a small range and whose direction can be adjusted with the control stick/circle pad, similar to Aura Storm. While a powerful attack, its small horizontal range creates blind spots. The meteors now deal flame damage, and during the attack the PK Starstorm sound effect from EarthBound plays. Between Ness and Lucas, both versions of the attack now function identically in terms of control, appearance, and damage output (including both characters having blue meteors).
The final smash now functions as a hybrid between the Brawl and Smash 4 variants, with the meteors being large and spawning in random positions, but the angle at which they travel can be influenced so that they are more likely to fly to the left or right of the stage. However since the spawn locations and angles are still ultimately random to a degree, the player has considerably less control than they did in Smash 4. Paula and Poo join Ness, while Kumatora and Boney join Lucas.
The differences between Ness' and Lucas' versions are much more pronounced. Ness' meteors will spawn at the top and centre of the stage, and will fly out at a range of angles, fanning outwards from an invisible "line" going down the middle of the stage. This line can be angled to the left and right, and newly spawned meteors will fan out from whatever angle the line is at the time, although the range of angles still varies significantly, limiting Ness' control over the meteors significantly.
Lucas' meteors instead spawn above Lucas, offset to the left and right, and will be angled such that they all cross through Lucas' position, and thus are angled inwards. The position that the meteors aim for can be moved both left and right, and up and down, and any spawned meteors will always cross through wherever the target position was when it was spawned, however the spawn position remains based on where Lucas is. In general, the player has more control over the path of Lucas' meteors than Ness'.
In addition, Ness' meteors are larger and deal more damage, but Lucas spawns a greater number of meteors, and as stated above, the player generally has more control over his meteors.
Ness as he calls down a host of damage-dealing stars in his Final Smash. Everyone on the screen must dodge like crazy to avoid getting hurt, but that's no easy task. This attack appeared in the Mother series, but it was Poo, not Ness, who wielded it. Perhaps Poo taught Ness the secret of the attack for use in Super Smash Bros. Brawl.
Lucas' Final Smash. He pulls a large group of stars from the sky and sends them smashing down among his enemies, causing damage to one and all. In Mother 3, this move was used by Lucas' traveling companion, Princess Kumatora. As Ness learned this move from Poo, Princess Kumatora taught Lucas when he learned he would appear in Super Smash Bros. Brawl.
In EarthBound, this PSI ability belonged to Poo, the crown prince of Dalaam and one of Ness's friends. Ness must have learned it especially for Smash Bros.! The attack makes a shower of meteors fall onto the stage, damaging any foes it hits. While the attack's in progress Ness is invincible and can adjust the meteor's trajectory.
You can adjust the direction of the stars in this attack! Rain stars down on multiple fighters, or be mean and pick on a single fighter. The choice is yours! If a star connects, it will deal ongoing fire damage, making it hard for opponents to return to the stage.