Mahishasura or Mahishan is a demon in Hindu Puranas. Goddess Shakti took the form of Durga and numerous other forms to fight and kill the demon. The story of the birth of Mashishasura is found in the Vamana Purana, Devi Bhagvatham and Skanda Purana.
The name Mahishasura because the demon had the head of a buffalo – Mahisha means male buffalo (bull).
An Asura king named Dhanu had two sons – Rambhan and Karambhan. Rambhan got a boon from Agni Deva that he will beget a son from the woman whom his mind really gets attached to.
Rambhan got attracted to a female buffalo in the abode of a Yaksha. He took the female buffalo to the netherworld (Patala). A bull got attracted to the buffalo and tried to mate with her. A fight ensued between Rambhan and the bull. The bull killed Rambhan in the fight.
The buffalo jumped into the pyer of Rambhan and killed itself. From the pyre appeared Mahishasura.
He then did intense penance and propitiated Brahma. He got the boon that he could only be killed by a female and by no other living in the universe.
Mahishasura then conquered all the known worlds in the universe. He unleashed a reign of terror. Goddess Durga devi then put an end to the Adharma propitiated by Mahishasura.