According to Theo Possible, the Zona Oscura was a "timeless expanse of mud, on the cusp of the conceptual and material planes". The mud was a form of bio-mass based on fiction that allowed lost ideas to exist, provide that it has a consciousness to live on.
The Zona Oscura Festival was located in the Zona Oscura, where a stray thought of Iris' lodged in her subconscious and tricked Iris and Panda into arriving at the Zona Oscura, attempting to devour Iris in a plan to become more real. (PROSE: Party Kill Accelerator!)