Shankh or Conch is an important symbol of Vishnu. After the great deluge, Bhagavan Srihari Vishnu was floating on the waters of deluge (Vatapatrashayi). Vishnu then created Brahma and asked him to begin the next cycle of creation.
Brahma who was given the freedom of creation soon became arrogant and forgot about his origin. His ego created numerous errors in creation.
To teach the arrogant Brahma a lesson, two demons named Lokan and Kantakan emerged out of the ears of Vishnu. They chased Brahma with an intention to kill him.
Brahma now realized his mistakes and took refuge at the feet of Vishnu. The ever-compassionate God forgave Brahma.
For their service, Vishnu offered Lokan and Kantakan boons. As boon, they asked Vishnu to fight them for a month and then kill them and give moksha.
Vishnu blessed the demons with the boon. He fought them for a month and then annihilated them. He then made the demons Nithyasuris – part of him. Thus Lokan was transformed into Shankha and Kantakan was transformed into chakra.
This story is found in the Pancharathra Agamam.