From here -
It was the month of Karthigai (between November 15th and December 15th), a Shukla paksha Ekadasi (the Ekadasi that falls before the full moon) day, our singer fasted the whole day and was awake that night (jagratha vritham). As usual, he left with his veena when he was accosted by a Brahmarakshasa named Somasharma who was totally famished. When a yagyam is not done as prescribed in the Vedas, the doer becomes a Brahmarakshasa and Somasharma was one of them. He was happy to find his prey. Nampaduvan, a devotee of Vishnu, did not fear death and was actually happy to satisfy someone’s hunger but before that he wanted to sing for the divya dampathis so he sought permission to complete his last wish. Who would believe that once allowed to go a human being would come back to be eaten by the Brahmarakshasa?
Nampaduvan made a list of promises. Seventeen oaths were made but the Brahmarakshasa was not convinced but the 18th one stumped him. The 18th oath was ”if I fail to turn up, I will accept the punishment given to one who equates Sriman Narayana with the demi-gods”.
Does scripture prescribe punishments for not declaring one or the other god supreme?