"Devi Mahatmyam" was written between 400 - 600 C.E. and is a part of the "Markandeya Purana," which is one of the 18 main puranas. It describes Goddess and her fight against evil, the demon, Mahishasura, and other demons. It is usually chanted during Navaratri, the nine-night celebration of Durga every autumn.
In yoga, it is believed that the fight of the goddess is actually the battle of the mind. Demons represent hurtful thoughts and feelings, such as greed, anger, jealousy, selfishness, criticism and so on. These thoughts may be harmful for an individual and their surroundings. Japa, yoga and meditation are used to invoke the Divine and to eliminate these elements of the mind.