The film's major plot centers around an unknown force that is slowly destroying the world. As such, the film is very dark and could be emotionally upsetting.
The fantastic ambient goes darker and more philosophically dense towards the end of the film.
Bastian being bullied at the start may be upsetting for kids who are going through that.
The appearance of G'mork the wolf is pretty frightening.
Atreyu's confrontation with G'mork near the end is very intense, especially when Atreyu dramatically challenges G'mork to fight him. There is also a lot of blood when Atreyu stabs him.