During the Galactic Civil War Han Solo and his strike team infiltrated Darth Vader's flagship, the Super Star Destroyer Executor. They got onboard the vessel presumably by the help of a nervous officer, who feared Vader's heavy-handed managerial style. The Rebel team successfully arrived to the Dark Lord's meditational chamber and stole his helmet. The theft caused the blast door of this section of the Executor to close, trapping the invaders. Admiral Piett and some stormtroopers were inside the sealed zone and they helped Vader to fight the intruders.
A possible place of the mission aboard the Executor
The mission appeared only in The Theft of Vader's Helmet scenario for the Star Wars Miniatures RPG-based tabletop game. The scenario suggested the Tractor Beam Reactor Coupling map to play on, which may or may not represent the actual place of the ambush. The scenario suggested Bossk, 4-LOM or Dengar for the Rebel strike team, but their participation in a mission led by Solo is highly unlikely, and should not be considered canon, since the Rebel team is assorted by the player.