The Societies of Dereth are factions that players will be able to join when the 100th patch goes live. For more information, see the Society System Overview on Turbine's website.
Rytheran's recent actions related to the Graveyard have caused a disturbance in the Ley Lines of the world - the rivers of mana that flow beneath the surface and give power to the sorcerers of the land. The three societies have formed to gain control of the ley lines and use them for various purposes. For more information on the three societies, please visit their pages: