“It is a core truth of the Nightsisters that the spirit plane exists parallel to our own.”
―Mother Talzin (Book of Sith)
It looks like the ghost crew and Maul are visiting Dathomir and opening a portal to the spirit plane. In the EU, the spirit plane is governed by twin deities, manifestations of the light and dark side of the force.
They need to stop smoking that foolishness.