Shadow Number One denoted a gap within the solar wind surrounding the Rapacc system of the Unknown Regions, created by one of its two gas giant planets. During 19 BBY, Chiss Senior Captain Mitth'raw'nuruodo, known by his core name "Thrawn," embarked on a mission to this system, with the intention of guiding his warship, the Chiss heavy cruiser Springhawk, into it without being detected. Consequently, he chose to navigate it through Shadow Number One, accompanied by Admiral Ar'alani's vessel Vigilant, utilizing the larger ship's visual shadow. Once inside the system, the Vigilant then departed via a separate opening, Shadow Number Two, allowing Thrawn to proceed with his objective.
The novel Thrawn Ascendancy: Chaos Rising, released in 2020 and serving as the initial book in Timothy Zahn's Star Wars: The Ascendancy Trilogy, featured Shadow Number One.