Maa Durga Blesses With The Light Of Knowledge That Leads To spiritual awakening 

“She Leapt Upon the great asura, Pinned his neck down With Her Foot, And Pierced him through With Her Spear."

- Chapter 3, Verse 40, The Devi Mahatmya

Maa Durga's Slaying Of mahishasura Is Symbolic Of the process of spiritual awakening. Maa Durga's Spear Symbolizes Knowledge, and being Pierced By It Symbolizes being Penertated By The Light Of Knowledge. The Penetrating Light Of Knowledge Forces mahishasura to emerge in his real form. The real form of mahishasura symbolizes the "fasle-self" or limited ego-awareness. To be able to progress towards the realization Of The True Self it is first important to discern the "false-self" and decapitate it.