Falias is the ruined City of the Gods. It is a divine place where the Gods are said to have lived before they decided to descend on Erinn.
To get to Falias, one must make a Portal by using either Shock or Awakening of Light while Brionac is equipped. One Falias Fragment will be used upon summoning the portal. To leave Falias, one must exit through the Falias Portal located at the entrance.
After you finish Generation 12, you can talk to the Falias Gatekeeper to change which goddess' powers you want to use, Morrighan or Neamhain. This will require five Holy Water of Lymilarks. You can also start the Falias Treasure Quest.