Shakti: Durga

Many scholars believe that tantra [or yoga] developed as an aspect of Hindu goddess worship.  It began as a purely oral tradition which relies on alternative types of techniques for spiritual progress - alternative to the usual cognitive methods that rely on intellectualization or on the intercession of a priest, etc.

For some people, the other senses (rather than cognition) such as sight and sound, provide a more direct way of gaining access to spiritual experience or advancement.  This type of personality is referred to in the Western tradition as a mystical one.
Now, in almost all cultures, the feelings that a person gets while experiencing the Spirit or 'God' or 'It' is thought of as derived from a kind of divine energy.  In the East (and many other places, too) that energy is thought of as female, in contrast with the 'Is-ness' or the Imagination, or the Being that is relatvely unchanging.

Judeo-christianity calls the source, God, but the female energy is understood.  In mystical Judaism (Cabalah or Kabbala) it is well known by the name of Sh'kina - this presence is especially felt on the Sabbath day when it/she is referred to as the Sabbath Queen,) and aalso of course, in Christianity where it emerged as Notre-Dame subsumed in the cult of Mary where its acknowledged presence is known as Mariolatry.

In the tradition of India, the activity of a god is called his shakti, and the god most often considered a yogi is Shiva, also referred to as Mahadeva, the Great God.
His consort is referred to as the supreme Shakti.   She is called Durga, the victorious, and her festival is a nine-night commemoration and invocation that takes place in the fall of the year.  This is Naava Ratri [the Nine Nights] which celebrates her 9 battles in the war against ashuras, the 'titans' aka anti-gods and the rakshasas, the malevolent ones opposing the devas or gods.

The scripture that recounts her various forms is the Markandeya Purana. It recounts how the devas [gods] were evicted from
heaven by the forces of the greatest of the ashuras.  They co-operated to provide the means of his destruction:  A bright
light radiated from the face of Vishnu in his intense anger. It merged with the lights of Brahma and Shiva. More energy in the form of blazing light came from Indra and the others. This light formed a mountainous fire which transformed into a beautiful and radiant female form.

That which was Shiva’s light, became her face. Yama’s light became her hair, and Vishnu’s became her arms. The moon, Chandra’s light formed her breasts. Indra’s light became her waist. Varuna’s light made her thighs, and the earth’s light created her hips. Brahma’s light became her feet; the sun, Surya’s light her toes and Vashu's her fingers.
Kubera’s light her nose. Prajapati’s her teeth. Fire, Agni’s light formed her three eyes. The light of the two sandhyas became her eyebrows. The light of Vayu, her ears. Other deities also contributed to her form and so she was called the Bright Goddess.

Later in the purana, there is another description of the assemblage of  the power (shakti) of each of the devas that has them each coming forth by means of the various and characteristic vahanas or mounts:
Brahma sent Brahmani in a gander/swan-drawn vehicle through the air, Maheshvari, the power of Shiva, also known as the great god, Shankar, came sitting on Nandi, the bull. The chaste power of Kartikeyya came on the peacock. The power of Vishnu came sitting on the eagle. Indra's power, Aindri came sitting on the elephant Airavat.

Then each god bestowed upon her his characteristic weapon: Shiva presented the trishul (trident) to her. Vishnu his discus. Varuna gave her the conch; Agni his spear. Maruna gave her a bow and arrows.  Indra presented her with his thunderbolt. Yama gave her the Staff of Death; Varuna his noose.  Brahma gave her the mala [rosary] and a water pot, and Kala gave her the stainless sword and bright shield. Visvakarman gave her the axe and impenetrable armor.  Kubera, lord of wealth  gave her the bottomless cup, and Shesha, a serpent-necklace of precious gems.

The Ocean adorned her with: A necklace of purity, upper and lower body garments that would never deteriorate, a crest jewel, a pair of earrings, bracelets, a half-moon pendant, armlets for her many arms, anklets, and rings for all her fingers. A wreath of unfading lotuses became her crown and garlands of them for her breasts; one also gracefully to adorn her hand. Finally, the mountain Himavat gave her the lion as her mount and many gems from his own store.

When Durga's form was complete, she let out a loud roar, followed by her laugh of defiance, again and again. The sound shool the very foundations of the world shake, and made the seas tremble.  The earth and its mountains vibrated responded with cries of. ‘Jaya - may you be victorious!’ The gods, too, when they saw her proceeding to the field of battle seated upon her lion.

On the first day, she is celebrated as Shaila Putri, Parvati, destroyer of rakshasas [demons] who rides the lion presented by her father Parvatha-raja.  When she comes to bathe in the Ganges, she finds all the gods singing hymns in her praise, and asks whom they are extolling.

Then from the sheath (kasta) of that body emerges Kaushiki, her most auspicious and beautiful form. But then Parvati's complexion darkens [kalika] and she becomes wrathful, or fierce.  Now she is Kali, of terrifying mien, shrieking and howling like a thousand jackals.

On the second day, she is again a maiden, the virgin Brahmacharani (Kanyakumari) whose essence is the purity of the water of the Himalayas.  When she visits the battlefield, from her pitcher she sprinkles the ashuras which renders these opponents of the Devas impotent.

Her third Durga form or avatar is Chandraghanta [Moon-bell] who rides a tiger.  Her implements held one in each of her eight hands are: the bell, trident, plough, conch, mace, discus, bow and arrow. This is her Maha Saraswati form, generous bestower of the light of civilization.  Sprung from the body of Parvati, she is the slayer of Nishumbha .

See a Nepali image of Durga doing battle.
Read the details of the battle. told by a Western devotee of Hindu guru, Satya Sai Baba

Her fourth day's avatar is destroyer of Mahishasura the buffalo.  She is seen seated on a lotus, of coral complexion and holding in her eighteen hands a rosary, axe (parshu), mace, arrow, thunderbolt, lotus, bow, pitcher, rod, pike (shuli), sword [khadga or talwar], shield, conch, bell, cup, trident, noose and the discus (chakra) called Sudarshana.

On the fifth day, she is Skanda-mata, mother of Lord Kartikeya, god of war.  With her son, Lord Subramanya, they are victorious over evil powers.

On the sixth night, again as Mahalakshmi, she manifests as goddess Kaathyayini who slays the demon Shumbha.   Shiva and other devas led by Agni praise her as Kathyayini, whose name is to be a powerful and auspicious mantra.

The seventh Durga form is Maha Kali in her aspect of  Kala Ratri, the terrible dark night for the demons.   From her third eye, this nemesis of the Rakshasas emerges as the Kali who severs the heads of two asuras, Chanda and Munda and offers them at the feet of the peaceful Durga earning the nickname, Chamundi.
A Rakshasa named Raktabija had been blessed with the boon that if one drop of his blood were to fall on the ground, another Rakshasa would immediately spring up.  He challenges Kali who defeats him by slaying him over her extended tongue drinking every drop of the blood.  She has ten faces, ten legs and holds the sword, disc, mace, arrow, bow, club, spear, missile, human head & conch.  She wears the garland of skulls and shines like blue jewel. Her vehicle is a human corpse.

On the eighth day, she resumes a pleasing form as a bride, Maha Gauri (Maheshwari) riding the white bull, Nandi her husband Lord Shiva's mount, and wielding a trident.
Along with Durga, various other forms of the Goddess appeared: Mahakali (Chamundi) riding on human corpse, Varaha (boar-form Vishnu's consort, the Sow with sharp tusks) riding on a buffalo, Aindri (Indra's wife) riding on the white elephant, Airavata, and Vaishnavi (Mahalakshmi) riding on a garuda, Narasimha (Lion-face, consort of Vishnu's Lion-headed form), Shivaduti of terrible prowess, Kaumari (Subrahamanya's shakti) on her peacock, Brahmani riding on the hamsa the gander or snow-white swan, all fully adorned with jewels.

Finally, on the Great Night, the ninth day (Mahanavami) she presides as goddess Durga from durgama (formidable.)  The ritual worship performed on the last night, especially in Bihar and Bengal where her religion flourishes today, is called Durga Puja.
For her devotees, She is the vehicle that transports them across the ocean of difficulties in the human existence.

The Shri Yantra [Skt.] the ritual diagram relating the union of male and female aspects, eg. union of Shiva and Shakti.