Dark Pit Staff
Dark Pit takes out his staff and fires a powerful horizontal shot, quipping "Goodbye!" or "It's time!" (in Japanese, either "貫け!", Penetrate!/Pierce through! or "一撃で決める...", I'll decide with one blow...) as he does so. Getting caught in the beam deals 1% damage, and the main attack deals heavy knockback along with an amount of extra damage which depends on how many opponents were already hit: 39% for the one closest to Dark Pit, 29% and 21% for the further ones.
In SSB4, the move is functionally a clone of Zelda and Sheik's Light Arrow, down to having the screen freeze and the camera zoom in on each character being hit. In Ultimate however, both Zelda and Sheik received new final smashes, so the move is now unique to Dark Pit.
Staff weapons are one of the nine weapon classes in Kid Icarus: Uprising. Though they lack in melee and homing capabilities, their range and increased damage from afar makes them ideal for sniping. In addition, the Special Attack for staffs is a giant laser beam, which both bears a striking resemblance to this Final Smash and needs careful aim to be at its most effective (like the Final Smash). The Dark Pit Staff, which is modeled after Dark Pit himself, inflicts a large amount of damage from a far distance, and its continuous fire is also powerful; however, its size and range are less than that of a normal staff.
Dark Pit uses this weapon in Chapter 6 if his location, chosen at random from four possible areas, is at the hilltop. If he is present in that area, he will attempt to snipe Pit from the hilltop once he enters. The effects and visuals in this final smash also look similar to the charge shot during air battles in Uprising.
The Japanese name of this Final Smash makes use of Dark Pit's nickname in the Japanese version of Uprising, being "Blapi", a shortened version of "Black Pit" (Dark Pit's Japanese name). The English-localized equivalent nickname is "Pittoo" (as in "Pit Number Two").
Wii U version
In Kid Icarus: Uprising, Dark Pit used this staff to snipe at Pit from a mountaintop. In that game, it did much more damage when used at a distance. In this game, Dark Pit uses it to launch a high-speed horizontal blast that goes straight through obstacles. It can hit multiple foes in a row, but only the first will feel its full force.
Note that the 3DS trophy refers to the weapon, rather than the Final Smash.
This weapon may be called a staff, but it actually does better shooting powerful shots at targets at long range. Like Dark Pit, this staff was created by the Mirror of Truth. If Pit were to use it, he'd probably feel lingering echoes of his clone's warped personality.
It might be called a staff, but this weapon can actually fire shots at enemies and is more effective at long range. It was originally wielded by Dark Pit, the copy of Pit created by the Mirror of Truth. Maybe Pit would get a sense of his clone's twisted personality if he used it.