Rhana Dandra is the Eldar name for the final great battle with Chaos. It is written about in the Asuryata, which predicts the destruction of the Eldar and their remaining gods. It is said that the battle will destroy both the materium and immaterium.
The Phoenix Lord Fuegan is said to be the one who will call the other Phoenix Lords together for the final battle and will be the last to die in the fighting, while Phoenix Lord Baharroth will also die his final death.
Since the Fall of the Eldar, a crystal tome bounded with chains of light has rested upon an obsidian plinth at the heart of the Black Library. As fabled events came to pass, so those chains faded one by one until the tome opened shortly before the opening of the Great Rift. Revealed within were writings said to have been written by Cegorach itself, telling of a final act that changed utterly the tale of the Fall. Instead of the Rhana Dandra, the final act tells of Cegorach's ultimate jest that would trick Slaanesh into expending all of her energies to save the Eldar instead of destroying them. How such an event could come to pass remains unclear.