Backlash / Aaron Allston.

Enhanced descriptions from Syndetics:

Repercussions from the dark side's fatal seduction of Jacen Solo and the mysterious plague of madness afflicting young Jedi continue to wreak havoc galaxy-wide. Having narrowly escaped the deranged Force worshippers known as the Mind Walkers and a deadly Sith hit squad, Luke and Ben Skywalker are in pursuit of the now Masterless Sith apprentice. It is a chase that leads to the forbidding planet Dathomir, where an enclave of powerful dark side Force-wielders will give Vestara the edge she needs to escape--and where the Skywalkers will be forced into combat for their quarry and their lives.

Meanwhile, Han and Leia have completed their own desperate mission, shuttling madness-stricken Jedi from Coruscant to safe haven in the Transitory Mists and beyond the grasp of Galactic Alliance Chief of State Natasi Daala. But the bold maneuver has intensified Daala's fury, and she is determined to shatter Jedi Order resistance once and for all.
   
Yet no greater threat exists than that which still waits in the depths of the distant Maw Cluster: A being of pure, ravenous dark-side energy named Abeloth calls out across the stars to Jedi and Sith alike. For some it may be the ultimate source of answers crucial to their survival. For others it could be the ultimate weapon of conquest. But for all, it is a game-changing--and life-altering--encounter of untold magnitude and a tactical gambit with unimaginable consequences.