Dev Kamrunag’s original name was Ratan Yaksha and he was a self-learned warrior. He would practice by keeping Lord Vishnu’s idol in front of him and he considered it as his master, the Guru. He got to know about the story of Mahabharat being fought in some far corner of Bharat and decided to take part in it. Brave and courageous he was, he decided to fight with the weaker party, which meant he was going to join the army of Kauravas.
Lord Krishan got to know about it and he decided to stop the self-learned warrior before he could reach the battlefield. Disguised as a yogi, Lord Krishna appeared before the Yaksha. He asked him about his journey and told him about the hardships being experienced by wounded soldiers. Ratan Yaksha listened to the story and it strengthened his determination. Lord Krishan gave him a tough test to find out the potency of his arrows and said, “I will be convinced if you can pierce every leaf of that enormous peepal tree with thy arrow.” When the Yaksha was preparing his arrow, the Lord plucked some leaves and hid them in his closed fists. To his surprise, the arrow pierced even the leaves in his fists. Then Lord Krishan asked him about his guru, upon which the Yaksha replied it to be no one but the almighty himself. And there Lord Krishan spotted an opening, transformed into his actual formless eternal form, and asked the young man for Gurudakshina, fee for services he had never offered to his student.
The yaksha could not resist and he had to offer what the Lord had asked for, his head. He gave away his head and asked the Lord to keep it alive until the Great War was over. The Lord immediately agreed and blessed him with the same, his head was brought to the Kamru Hill and today it is known as Kamrunag Temple.
Legend also says that the head was kept at the Nalsar Lake in Mandi district but because of climatic problems faced by the head, it was shifted to the top of the hill, from where he could see the greatest wars of all time, live and exclusive.