Derian was an accomplice of Pasiphae who helped clear the way for Medea to steal the Palladium from the chamber inside the palace. He was captured by the Atlantean guards and questioned via torture in order to discover what had been planned.
To protect his identity, Lord Sarpedon poisoned Derian whilst in chains, and it was believed he had died because the guards had pushed his torture too far. (A New Dawn Part 1)