"No, that would be something, this was... Nothing! And it got bigger, and bigger, and bigger..." - The Rock Biter, The Neverending Story.
The Nothing is a power which destroys Fantasia one by one by absorbing it. The Nothing gives people the desire to surrender themselves to oblivion by jumping into the nothing. Gmork said that the Nothing would be a chance for beings of Fantasia to come to the human world, but as lies. The Nothing was created by the manipulators; in fact, it's the emptiness that remains when people stop to have fantasy. The Nothing succeeds in destroying Fantasia, however, Bastian creates a new version of it.
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 Rockbiter 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 revealed 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.
The Manipulators are the power behind the Nothing and Gmork's masters. Their goal is to destroy Fantastica in order to replace fantasy with lies to manipulate and control the human race.