C-deck existed within the Undercity that was part of Sundari, which itself was located on Mandalore. In the course of the Siege of Mandalore, a ground-based contingent from the 332nd Company received orders to bolster the defenses of C-deck; however, they were caught in an ambush orchestrated by the rogue Sith Lord Maul, resulting in significant losses. Following this assault, Ahsoka Tano, the one-time Padawan, along with Rex, a Clone Commander, went to the ambush site. After discovering the events that transpired, Tano instructed Rex to ensure that C-deck's defenses were adequately strengthened.
The initial reference to C-deck occurred in the Star Wars: The Clone Wars Season Seven episode entitled "The Phantom Apprentice."