We don’t really know anything about the Dark Star. We have discovered that it’s what The Return is looking for.

The Dark Sun or Dark Star is the dead star/sun of Athas. the people of Athas drained and killed their sun in their quest for expansion. The Return want to restore Athas likely to have access to the ability to draw power directly from living things, and also to bring back a faction that could rival the Koalitio

The Return are looking for a ritualistic way to achieve this. The Astra Exteri posess the Star chart to Athas and the Archivist’s Key which Galenden Hem believe together could cause this return.