He then considered himself immortal and started freely roaming in the garden of Yaksha where he saw a female-buffalo and fell in love with her. To express his love, Rambha disguised in the form of a male-buffalo and copulated with the female buffalo. However, soon after that a real male buffalo discovered Rambha mating with the she-buffalo and killed him. It was due to Rambha’s inflated ego that killed him, out of which he has not asked his death to be spared from the wrath of animals. As the pyre of Rambha was organized, the female-buffalo, who was copulated with him jumped into the funeral pyre of Rambha to prove her love. She was pregnant at that time. Thus, demon came out of the with the head of a buffalo and human body and he was named Mahisasura (the buffalo headed demon).
Mahishasura was extremely powerful. He defeated the gods and the demons and acruierd power over the entire world. He even won over the heaven and threw devtas outside it. He captured the throne of Indra and declared himself to be the the lord of the gods. The gods led by Brahma approached Vishnu and Shiva and evaluated them of the situation. In order to save the Gods, the three supreme deities emerged a light of anger, which combined to the take the shape of a terrible form and this was Durga. All the gods then granted this Goddess of power with all the supreme weapons they had. This is why; Durga is called the brilliance of all the Gods.
When the goddess was seen by Mahishasura, he was mesmerized by her beauty. Her then fell in love with her and proposed to marry her. The goddess said she will marry him, if he defeated her in the battle. Then began a scary and terrible battle between both of then which continued for nine days. Finally, on the last day, Durga took the form of Chandika and stood over the chest of Mahishasura and smashed him down with her foot. She then pierced his neck with her spear and cut off his head off with her sword. It is the day when Vijayadashmi is celebrated.