Panchajanya (IAST: pāñcajanya) is the Shankha or conch of the Hindu god Vishnu. As per Valmiki Ramyana, Purushottama (Vishnu) killed a Danava named Panchajana on a mountain named Chakravan constructed by Vishwakarma and took away conch shell known as Panchajanya from him.
As per Harivamsa, Krishna, the avatar of Vishnu, is described possessing a shankha called Panchajanya, one of his four attributes together with the mace Kaumodaki, the disc-like weapon Sudarshana Chakra and a lotus flower. The conch was used during the Kurukshetra War. Panchajanya or Shankhasur was an evil sea demon who lived in a colossal conch shell in the deepest depths of the Prabhasa ocean or was a demon in the form of a conch shell. He kidnapped the son of Sandipani, the guru of Shri Krishna, Balarama and Sudama, and imprisoned him in the conch shell. Upon completion of their studies, Krishna, Balarama, and Sudama persuaded the teacher to ask for the preceptor’s dakshina (fees) of his liking. Sandipani asked for, as his dakshina, the restoration of his child. Krishna became enraged when he heard about the kidnap and plunged into the sea to rescue the son of Sandipani. Krishna successfully slew Shankhasur and took the conch shell for himself. He then named the conch shell after the Demon. Whenever Krishna blows from the shell it foreshadows the death of his next opponent.
Basically, the most prime god Shree Hari Vishnu is said to be quad-armed and in one of his hands he holds the panchajanya conch, whose a copy, like other shastras, has been transferred to lord Krishna on earth from the so-called heaven- Vaikuntha dhama. Lord Krishna found him in a deep sea. A demon used to sleep in this conch and thus was destined to be killed by holy god, at the time of receiving the conch.