Why and How Lord Shiva Killed the Tripurasuras?

According to the Shiva Purana – Rudra Samhita – Yuddha Khanda – 1 to 10, Lord Shiva Killed the Tripurasuras: After the destruction of the demon Tarakasura by Skanda, the three sons of Tarakasura, namely Tarakaksha, Vidyunmali, and Kamalaksha performed severe penance in a cave of mount Sumeru. Lord Brahma appeared before them and told them to ask for a boon. They asked for three cities (puras) that are indestructible by the gods. Tarakaksha asked for a golden city, Kamalaksha asked for a silver city and Vidyunmali asked for a steel city. The cities can only be destroyed by Lord Shiva by a single amazing arrow when they all join together. Lord Brahma granted the boon and got the three cities built by Mayadanava.

Getting the boon from Lord Brahma, the Tripurasuras defeated the devas. However, they were engaged in the worship of Lord Shiva within their cities. The devas prayed to Lord Shiva to destroy Tripurasuras, but since they were engaged in his worship, Lord Shiva declined to do so. The devas took refuge in Lord Vishnu. Lord Vishnu in order to deviate the Tripurasuras from the worship of Shiva, created a clan of shaven-headed ascetics. The ascetics through their misleading preachings made the Tripurasuras gradually stop the worship of Lord Shiva.

The devas again prayed to Lord Shiva to vanquish the Tripurasuras. Lord Vishnu told the devas to utter the “Om Namah Shivaya” mantra one crore times. When the devas did so Lord Shiva became pleased. He told the devas to make a special chariot for waging a battle against the Tripurasuras. Vishwakarma was given the task to build this special chariot. Vishwakarma created a special chariot representing the entire universe. Sun and Moon were its wheels. The six seasons were the rims of the wheels. All the elements of time were part of the chariot. All the key mantras were part of the chariot. In short, it was an amazing chariot. Lord Shiva boarded the chariot and when the three cities joined together, he burned them with a single arrow thus destroying the Tripurasuras.