Betwixt and Between

Betwixt and Between is a Corridor of Darkness that leads from the Basement Hall of the Old Mansion to the Dark City. It serves as the means to first access The World That Never Was.

Setting
Betwixt and Between is simply one corridor surrounded by walls of swirling colors that fade from orange near the entrance to Twilight Town to dark blue near the entrance to The World That Never Was. The floor and walls both have Nobody symbols flowing across them.

Kingdom Hearts II
In order to catch Roxas at the simulated Old Mansion, Axel creates a Corridor of Darkness from the Dark City to the Basement Hall, but never closes it. After Axel is defeated, the corridor still remains, but if Roxas investigates it, it simply says "It looks like it leads somewhere, but the power of darkness prevents entry."

When Sora arrives at the Simulated Old Mansion, he finds the portal the Axel left behind. Then, he, Donald, Goofy, and King Mickey enter on their way to The World That Never Was. As soon as they enter, though, they are attacked by a swarm of Dusks. After fighting off some, they are about to give up when Axel appears and tells them to get going before being attacked himself.

Sora saves him by slaying a few Dusks, and then Axel reveals that he kidnapped Kairi, and that Saïx has her now. Sora and Axel are then surrounded by a huge swarm of Dusks, but Axel eliminates them all with one attack. However, this takes Axel's whole being, and this causes him to start "fading away". Axel then apologizes for what he did to Kairi, reveals where she is, and opens a portal to the World That Never Was before disappearing. Sora and co. then go through the portal into the World That Never Was.

Trivia

 * If Kingdom Key, Oathkeeper, or Oblivion is equipped in this area, the summoning animations will not be shown and the Keyblade will appear out of nowhere, just like it did in the first game.