Planet of Twilight

Planet of Twilight is a novel by Barbara Hambly.

Publisher's Synopsis
Nam Chorios is a barren backwater world - once a dreaded prison colony, now home to a fanatic religious cult. It is here that Princess Leia has been taken captive by a ruthless and charismatic warlord bent on destroying the New Republic. Meanwhile, Luke lands on a mysterious planet in search of his lost love, Callista, only to discover the Force is his own worst enemy. But worst of all, as Han, Chewie, and Lando leave Coruscant on a deperate rescue mission, a strange life-form, unlike any the galaxy has ever seen, awakens... a life-form so malevolent it will destroy everything - both Empire and New Republic - on its path to domination.