"No, that would be something, this was... Nothing! And it got bigger, and bigger, and bigger..." - The Neverending Story
In the first half of the Michael Ende's novel, The Nothing is revealed to be a mysterious and dark parasite with the sole intention of running the magical universe of Fantasia into extinction by consuming its very existence. In the film adaption of the novel The Nothing is depicted as a raging storm that takes things away.
When compared to a hole the Rockbitter replied that a hole would be something, but there was nothing. Later on when Atreyu attempts to glance at The Nothing his eyes hurt, simply because his brain was not able to comprehend what it is he saw. It is eventually revealed that some unknown entity is behind the Nothing, though the identity of this entity is never revealed.
In the novel, the Nothing is reveiled to be tied to the human world or more accurately the Nothing is created by humans' lack of imagination and reluctance to read books. The Gmork implies that all Fantasians lost to The Nothing are reborn in the human world as lies.