Limbo is a realm that existence between Paradise (above it), and Hell (below it). Like Earth, Limbo is "neutral" - being neither good nor evil, but sightly both in equal parts. Limbo is a place with no visible ground, save for a maze of walkways, and strange objects scattered across the endless landscape. Strange creatures dwell in this realm, like the Pherox.
In the cartoon, Reina - a former Angel, who broke the V.E.T.O. in a serious way - was imprisoned in a special jail in Limbo, and for it, became a "Neutral One." Arkan and Temptel where able to find the prison by casting Sin (Gilt) Fly, to find Maliki's tracks, as he often traveled trough Limbo to places on Earth.