Complex sentence: She is part of the story behind the Jewish holiday of Purim one of Judaism's happiest festivals, because her demise opened the way for Esther, the eventual saviour of ancient Persia's Jews, to be chosen as Ahasuerus's next queen.
Split into simpler sentences: She is part of the story behind the Jewish holiday of Purim one of Judaism's festivals. Her position was superceded by Esther, the reputed saviour of ancient Persia's Jews, who was as Ahasuerus's next queen.