Dark Apprentice is the second novel in The Jedi Academy Trilogy by Kevin J. Anderson; many of the events of this novel are also described from the perspective of Corran Horn in I, Jedi.
While the New Republic struggles to decide what to do with the deadly Sun Crusher—a new doomsday weapon stolen from the Empire by Han Solo—the renegade Imperial Admiral Daala uses her fleet of Star Destroyers to conduct guerrilla warfare on peaceful planets. And now she threatens Mon Calamari,[1] the watery homeworld of Admiral Ackbar and important shipyard for the New Republic. But as the battle for a planet rages, an even greater danger emerges at Luke Skywalker's Jedi academy on Yavin 4. A strong but untrained student named Gantoris delves dangerously into the dark side of the Force and unleashes the spirit of the ancient dark side master Exar Kun, who instructs him in the creation of a unique, three crystal lightsaber that can be focused to extend beyond the normal length to that of a spear. The spirit of Exar Kun also tries, with disastrous consequences, to entice him toward the dark side. Although unsuccessful, this sets the stage for another of Luke's students, Kyp Durron, to face the same choice as Gantoris.
When he does get to the Jedi Praxeum, he starts learning the ways of the light side of the Force, astonishing all the other students with his extraordinary talent and his aptitude for Force-training; unfortunately, he also starts learning of the dark side of the Force after Gantoris' death (Gantoris had been corrupted by the spirit of Exar Kun, but turned against him; in revenge, Exar Kun burned Gantoris alive). Exar Kun's spirit began tempting Kyp. Kyp's power increases considerably, and he is tainted by the dark side and the lies Kun told him of the history of the Jedi Order. After a violent disagreement with Tionne over ancient history (involving the fall of Exar Kun), Kyp assaulted Skywalker; with the dark aid of Kun, Kyp ripped Luke's soul from his body, placing him into a coma. With his strength in the Force, Kyp reached out and guided the Sun Crusher from the heart of the gas giant, Yavin, and set out on a mission of vengeance against Admiral Daala and the Imperial Remnant. He knows that his brother, Zeth Durron, is still part of the Empire and he sets course for Carida, intending to rescue him by force if need be.
Dark Apprentice - Wikipedia