The correct answer differs. It is worthy to note that although Atreyu was in a journey to save Fantasia (Fantastica in Michael Ende’s novel), he was really saving the CHILDLIKE EMPRESS from drying. However, it can also be said that Atreyu was in fact saving Fantasia because if he was not going to find a cure for the childlike empress, the formless entity called “The Nothing” will destroy the magical land of Fantasia.
The Neverending Story was published in German in 1979 and translated to English by Ralph Manheim in 1983 and it covers the story of Atreyu and Batsian Balthazar Bux. There were a few changes between the book and film – one of which is the change in the name of the magical land from Fantastica to Fantasia. Which brings out to the conclusion that the question being asked is talking about the film version.