The Dark Jedi found strong emanations of the Force from the Death Star debris. Vader immediately realizes that this is connected to Obi-wan, but is also forced to admit that the boy is somehow involved as well and directly connected to his former master. Travel to the Nightsister's Nightspider cult Cave on Dathomir and retrieve a Force crystal from a cave there.
Head to the Nightspider Cult Cave on Dathomir. Inside there will be a force crystal, all you need to do is click on it and the quest updates