Betwixt and Between is a strange realm that connects to 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.
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 City That Never Was, in the Alley to Between.