Mahakaali - Anth Hi Aarambh Hai

Mahadev's Shocking Decision (1x23)

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Mahakaali arrives at their hideout with the decision to kill Shumbha and Nishumbha. She kills Nishumbha and then decides to go after Shumbha. But Mahadev stops her from killing him and asks her to leave him alone. What is the reason behind his decision?