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. Another section states 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. It is unknown what other races will take part in this, or even where it will occur, but it is a strong theme in Eldar mythology.