Star Trek: New Frontier, book 08 : Dark Allies
Peter David1999

Many years ago, a bizarre alien lifeform known as the Black Mass consumed and destroyed an entire solar system in what was then the Thallonian Empire. Now the Black Mass has returned and its target is Tulan IV, homeworld of the fearsome Redeemers. Faced with near-certain destruction, the Overlord of the Redeemers is forced to turn to an unlikely ally: Captain Mackenzie Calhoun and the Starship Excalibur. Busy coping with the return of his rebellious son, Calhoun is none too eager to come to the aid of his despotic enemy; but when innocent lives are at stake he has no choice but to confront the implacable Black Mass. But how can one solitary Starship hope to turn back a force capable of consuming entire suns?