The Twenty-First was a unit that fought for the Rebel Alliance throughout the Galactic Civil War. This group saw action during the Mid Rim Retreat, and they were sent to the world of Bestine IV in 3 ABY. Their mission was a coordinated attempt to divert the attention of the Galactic Empire's military, giving the Alliance Fleet the time it needed to complete its withdrawal from the Mid Rim. The 61st Mobile Infantry of the Alliance heard about the Twenty-First's deeds, and Sergeant Maximian Ajax told First Sergeant Hazram Namir about it.
Alexander Freed made reference to the Twenty-First in his novel, Battlefront: Twilight Company, which was published in 2015.