The asura king Mahisasura was a combination of both an Asura and a mahisha ("water buffalo") with a trident. As an Asura, Mahishasura wanted to wage war against the Devas, as the Devas and Asuras' were in conflict. To make himself invincible, Mahishasura performed austerities (tapas) to Brahma, propitiating Brahma to confer upon him immortality. After which Brahama refused him the boon of immortality; instead, he gave him a boon such that his death would happen only at the hands of a woman.
After which Maa Durga took birth to kill the asura king Mahisasura.
Goddess Durga was incarnated with ten arms, each having different warriors with Lion as her vehicle and possesses the power to destroy the demon rakshas Mahisha.
Mahishasura ruined the life of humans and conquered Earth (Prithvi Lokam) as well as Deva Loka (Swarga Lokam) by attacking Lord Indra, the King of Devas.
All Gods and Goddess approached Lord Vishnu for a solution. Thus the powers of Trimurtis - the Trinity of Lord Brahma, Vishnu and Mahadeva - joined together for the creation of Goddess Durga (Mahamaya).
Durga Maa went to fight with Mahisha and at the end of the fierce fight she got victory over Mahishasura. Thus Durga got the name Mahishasuramardini (The one who killed demon Mahisha).
After killing Mahishasura, Goddess not only saved the world from the demon but later she came to be known as the Mother of the Universe who embodies the primeval source of all power.