The Palladium was an artefact found deep inside The City of Atlantis, heavily protected under the palace. A prophecy once told that if the Palladium was removed from Atlantis, it could fall to any invasion.
Pasiphae planned for Lord Sarpedon to travel to Atlantis with another accomplice, Medea, hidden inside a chest. During one night, Medea was released from the chest and travelled deep in the palace to locate the chamber's entrance. She stole the Palladium, causing Poseidon to release his anger on the city. Whilst being chased by Atlantean guards, she escaped by jumping onto the back of a winged creature. Arriving at Pasiphae's camp, she gave the Palladium to Pasiphae. However Jason, Pythagoras and Hercules managed to sneak into the camp and steal it back. Perused by the Colcheans they ran into a cave. (A New Dawn Part 1)
After all a long journey, they managed to return the Palladium to Atlantis and gave it back to Ariadne. She showed it to the forces to restore there faith, it was presumably restored to its rightful place. (A New Dawn Part 2)