Durga is combined form of the 3 goddesses - Saraswati, Mahalakshmi and Parvati.
She had killed the mahishasur after 9 days of intense fighting. As chamunda she had killed the demons Chanda and Munda.

Today, I will share a very popular mantra chanting which a sadhak will get rid of all kinds of sadness, worries, anxieties and apprehensions one may have.

When times become tough and the sadhak cannot understand what to do, when someone is suddenly caught by a fearful diseases, when the enemies are becoming so strong that you life comes in question, when poverty is coming to grasp you and your family and when days become so bad that you even start having suicidal thoughts, at such moments, by chanting this mantra, you will get rid of all kinds of negativity, all kinds of sadness, all kinds of diseases and all your negative apprehensions will turn false and you will  see positive events happening in life.

The mantra is tried and tested. And there is no meaning of doubting this mantra. No success is gained if someone doubts this mantra.
The mantra must be chanted early in the morning in front of a chamunda yantra or durga yantra.
The mantra must be chanted using a red hakik or a red chandan mala only.
At Least 5 malas must be chanted daily if someone wants to see quick results. Those who lack of time must chant at least 1 mala daily.
One can chant this mantra as long as they want.

Mantra -

|| सर्वमङ्गलमाङ्गल्ये शिवे सर्वार्थसाधिके ।
शरण्ये त्र्यम्बके गौरि नारायणि नमोऽस्तु ते ॥
Sarva Mangala Maangalye Shive Sarvaartha saadhike |
Sharanye Traiyambake Gauri Narayani Namostu te ||

Meaning:
Who is the Auspiciousness in All the Auspicious, Auspiciousness Herself, Complete with All the Auspicious Attributes, and Who fulfills All the desires of here devotees,
Who gives refuge to those in trouble, With Three Eyes and a Shining Face; Salutations to our mother Narayani.