The kingdom of the Dead where Arthur is taken by barge?

Upon having suffered a mortal wound from Sir Mordred, Arthur is taken on a barge accompanied by The Lady of the Lake and three queens. They are his sister, Queen Morgan le Fay, the Queen of Northgalis and the Queen of the Wastelands. Though never seen again, there is a legend that he is not dead and will come again when a great king is needed.