The Buna Werke camp was a factory to which concentration camp Jews were sent if they seemed fit both physically and mentally to go work there. It was also known as Auschwitz III. We know that Elie Wiesel and his father were saved from execution by being transferred there with the condition that they will be submitted to hard hours of labor, starvation, brutal treatment, and systematic military-style discipline.
Buna was no different than Wiesel's previous camp. By a suggestion of a fellow inmate he lied to the SS who were selecting the Buna workers and said that he used to be a worker previously and that he was 18 years old. This qualified him to go to Buna.
However, the conditions there were quite dire. Wiesel spent eight months there enduring some of the worst treatment he ever witnessed. What is worse, he witnessed in Buna the first trial done by Jewish tradition against God. Three fellow Jewish men declared God guilty of the situation that was going on. It is an ancient tradition that he witnessed in his adolescence, which is quite a hard thing to experience.
After Buna, Elie was taken to Buchenwald. It was during this time that his father died of dysentery and starvation.