Holocaust

The Holocaust was a terrible event that was part of World War II and was caused by the Nazis.

Little is known about the Holocaust. It took the life of Erik Magnus's family. He remember the loss for the rest of his life, using it as reason for mutants to dominate humans.

Background
The Holocaust was not named nor elaborated on. It was simply implied on.

In the Comics
Magneto was originally a man during the Holocaust. As time went on it became more unbelievable that he could be a man during the Holocaust and still remain a threat to the X-Men. He was then de-aged to a boy. The series maintained the latter explanation.