Betwixt and Between

Betwixt and Between is a strange realm which serves as a passage between the simulated Twilight Town and The World That Never Was.

Sora found Betwixt and Between inside the mansion in the simulated Twilight Town. Roxas had also seen it previously, but the power of darkness prevented entry. Kairi and Pluto used it to escape Axel and flee to Twilight Town.

While exploring the simulated mansion, Sora discovered the portal to Betwixt and Between in the Basement Hall. Upon entering, the group had to fight a large number of Dusk Nobodies, until they were saved by Axel. Using every ounce of his strength, Axel unleashed a massive explosion which wiped out the Dusks, but also resulted in Axel's death. Before fading away, Axel opened the portal to Organization XIII's stronghold, dropping Sora into The World That Never Was, in the Alley to Between.

It appears to be what a Corridor of Darkness opened by a Nobody looks like on the inside; that or simply one leading to an in-between world.

Trivia

 * The battle menu used in Betwixt and between is the same as the one used in Kingdom Hearts (game)