Poseidon's Conch Shell is one of ten Godly Possession found in God of War III.
Using it during a following playthrough will disable new trophies from being unlocked.
Description
This Possession grants infinite Magic (The same ability given by The Bonds of Ares in Ghost of Sparta), allowing Kratos to wield his devastating abilities with impunity.
Location
In Poseidon's chamber, where the princess is found, in an alcove to the right of the portal.