A demon, the reborn Sudama, married with Tulsi, killed by Shiva.
It is a similar story as Vrinda and Jalandhara. In both stories Vishnu is cursed to become a salagrama.
Read about Shankhachuda here
http://hindumythologybynarin.blogspot.com/2013/10/shankachuda-demon-killed-by-shiva-and.html