Nanclus was the identity used by a Romulan antagonist in Star Trek VI: The Undiscovered Country.
The book Cast No Shadow establishes that the man known as Nanclus was an agent of the Tal Shiar and was using the name Nanclus as a cover identity. His real name was not revealed, nor was it revealed if there was a real Nanclus that he had earlier replaced.
In the 2290s Nanclus served as the Romulan Empire's Ambassador to the United Federation of Planets. When the Klingon moon Praxis exploded Nanclus feared that peace between the Klingons and the Federation would be against the interests of the Tal Shiar. He joined the conspiracy with the Federation's Admiral Cartwright and Lieutenant Valeris plus the Klingon General Chang among others to sabotage the peace deal between the two powers.
Due to the efforts of James T. Kirk and the crew of the USS Enterprise (NCC-1701-A) the conspiracy was defeated. Kirk kept the Federation President from being assassinated, and Nanclus was arrested.
Following the opening session of peace talks on Khitomer, Nanclus was arrested and returned to the Romulan Empire to face charges of treason. Nanclus was sentenced to death and as far as the Romulan general population, Federation, or the Klingons had been killed by a weapon set to disintergrate. This allowed Nanclus to slip away and undergo cosmetic alterations. After those alterations were done he returned to his role as a Tal Shiar section chief.
In 2300 Nanclus was involved in further efforts to destabilize the Klingon government by supplying Kriosian terrorists by providing them isolytic subspace weapons. The Imperial Intelligence agent Kal and Valeris were able to prevent the destruction of Qo'noS by the terrorists. Kal then set out to kill the Tal Shiar section chiefs during one of their get togethers. The man formerly known as Nanclus was the last to die, with the last words he heard being the Klingon expression that traitors cast no shadows.