From the Super Mario Wiki
Broken into three parts, the players have to collect Dragoon Part A, the nosecone, Dragoon Part B, the hull, and Dragoon Part C, the tail. Any player can collect any of the three parts of the ship, but when one player has all three at the same time, they can use it to crash into the stage, knocking out all players who are hit by the Dragoon. When players are damaged while holding the Dragoon parts, they may lose a part, meaning that an opponent can easily snag a lost part. The Dragoon parts don't always appear in every single match.
Anyone hit by the Dragoon will almost always be KO'd, but certain factors (such as walls, metallic, invincibility, etc.) might prevent the targets from getting KO'd.
Role in Subspace Emissary
In the Subspace Emissary, the Dragoon appears once, and acts similar to the Warp Star (as Kirby's vehicle). In the scene when Ganondorf and Bowser just blew up the Halberd, after the explosion happens, almost all of the heroes of the game arrive in their ships (Fox McCloud in his Arwing, Samus in her ship, Olimar in his ship, and everyone else in Captain Falcon's ship). They all continue shooting the enemy ship and the enemy's ship also fires back. However, the enemy's ship is so large that the aircrafts don't do much damage to the large ship. Just when it seems the heroes are losing, Kirby comes flying in on the Dragoon and flies straight at the ship's cannon. He flies through it, destroying the cannon in an explosion. The other ships then keep firing at the ship and soon, the whole ship is on flames. However, Ganondorf and Bowser leave into the Subspace right before the whole ship explodes.
Super Smash Bros. for 3DS and Wii U
Trophy Information from Super Smash Bros. Brawl