Star Trek The Next Generation, book 31 : Foreign Foes
Dave Galanter1994

The Hidran race and the Klingon Empire have been at each other's throats for seventy years, and Captain Jean-Luc Picard has been asked to do the near-impossible: take the USS Enterprise to the planet Velex to mediate a treaty that will end the conflict between these two aggressive species. Things get off to a rocky start -- then turn deadly as the Hidran ambassador mysteriously dies, and kills a Klingon delegate as a last act of vengeance.

When Lt. Worf is charged with the ambassador's assassination, and Commander Riker and Counselor Troi are trapped far below the surface of the planet, Captain Picard must not only act to save the Hidranas and Klingons from each other, but to save his Klingon officer from a hideous death sentence...