When Hiranaksha was killed, Diti was deeply grieved and Nirhad Daitya, the maternal uncle of Prahlada, decided that the root cause of all the tribulations for the Daityas were Vedas and Mantras. As Brahmanas were the performers of Yagnas and the Vedic Rites, the Demon took the form of a tiger in a forest nearby and during the course of a Yagna at Kasi which is the Center of Vedic activities, he used to kill as many Brahmanas as possible in the nights and after the success of this experiment, more Daityas followed suit.But, when several Brahmanas assembled worshipping a Siva Linga on a Sivaratri, a tiger appeared and Lord Himself gave a powerful blow and terminated the Daitya in the form of another tiger.
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http://www.kamakoti.org/kamakoti/details/shivapuranam16.html