The Twilight Cage (トワイライトスペース,Towairaitosupēsu?, lit. "Twilight Space"), also called Argus' Cage by Nestor, is a alternate dimension that appears in chapter 7-10 of Sonic Chronicles: The Dark Brotherhood. It is a mysterious prison dimension which has been inhabited by several alien races throughout the ages, along with the Nocturnus Clan.
The Twilight Cage appears very much like the inside of a space nebula: spiraling, indigo gas against a backdrop of distant star lights. The only solid ground in the Cage comes in the form of planetary fragments torn from other worlds, including the Nocturnus Clan's city-fortress, the Nocturne.
The Cage has been (unwillingly) inhabited by many races during its history. The first inhabitants of which any records remain are the "Precursors", who shared the dimension with a variety of unnamed species. When Sonic and his friends arrive in the Cyclone during Sonic Chronicles, the eldest race residing there is the Kron. Alongside them are the Zoah, the N'rrgal, the Voxai, and the Nocturnus Clan echidnas.
As of Sonic Chronicles: The Dark Brotherhood, the exact nature and purpose of the Twilight Cage remains shrouded in mystery. Sonic helps Nestor the Wise decipher some clues by gathering Precursor tablets during their travels. According to Nestor's translation, the Precursors believed the Cage to be a prison for dangerous and powerful races, designed by a godlike being called Argus. The validity of this hypothesis has yet to be determined.
Time flows at a different pace inside the Twilight Cage. A day inside might translate to mere seconds in some 'normal' dimensions; while for others, years might pass on the outside. It is the latter arrangement which holds sway between the Twilight Cage and Earth; when Sonic and friends leave at the end of Sonic Chronicles: The Dark Brotherhood, they discover Dr. Eggman had enough time during their absence to rebuild Metropolis. Additionally, the Nocturnus Clan has been imprisoned there for over 4000 years and have not aged a day.
The entire dimension of the Twilight Cage operates on a different frequency from Sonic's dimension, meaning that the Twilight Cage can only be entered and left through the use of warp generators or wormholes. For the creation of wormholes into the Twilight Cage, however, it requires a level of power that only the Master Emerald can generate.
The Twilight Cage could be derived from the Twilight Zone from the Archie comic series. Issues printed during the release of Sonic Chronicles: The Dark Brotherhood started to use the name "Twilight Cage" to refer to the Twilight Zone, meaning that, in the Archie comics universe, the two are one and the same. The characters Kragok and Lien-Da even blamed the assassination of their father, Luger, on the Zoah, one of the alien races exiled to the Twilight Cage.
The sectors were named after Scylla and Charybdis, referencing an idiom that derives from Greek mythology.
↑The Codex entry for the Kron states that they are at least believed to be the oldest race in the Cage. The timeline of the other races' arrivals can be partially deduced from other comments within the game. The N'rrgal Queen states "Ever since we arrived, the Zoah have tried to conquer ussssss" (Sonic Chronicles, Chapter 8), which implies that the Zoah arrived before the N'rrgal. The Nocturnus Clan obviously arrived most recently; so the only race without any concrete place in the timeline is the Voxai. The best info comes during the Twilight Conference, when Thebes states that "We have fought each other for thousands of years" (Sonic Chronicles, Chapter 10); so the Voxai have at least been there for that long.
↑From Sonic Chronicles: The Dark Brotherhood, end sequence.