In that year, he was involved in a conspiracy to sabotage peace talks between the Federation and the Klingon Empire. While in a meeting with the Federation President after the arrest of James Kirk and Leonard McCoy, Nanclus lent his support to "Operation Retrieve," the proposal to infiltrate Klingon space and rescue the hostages, put forth by Nanclus' fellow conspirator, Colonel West. Later, Nanclus attended the Khitomer Conference. After the crew of the USS Enterprise-A learned of his involvement in the conspiracy, and successfully prevented the assassination of the Federation President at the conference, he was detained by Starfleet. (Star Trek VI: The Undiscovered Country)
Nanclus was played by Darryl Henriques.
It is unclear whether Nanclus' conspiracy was sanctioned by the Romulan government, although that would be consistent with its history.
Author David A. McIntee once proposed that Nanclus was actually the initiator of the conspiracy, having secretly put the idea that the Federation were the true threat to the Klingon Empire into General Chang's mind, due to Romulan concerns that the Klingons and Federation were about to form a pact. McIntee went on to characterize Nanclus as "almost certainly the true villain of the movie, and probably the most forgotten of the Trek villains." (Star Trek Magazine issue 149, p. 64)
The novel Sarek gives the Romulans a more prominent role in the conspiracy, revealing that Admiral Cartwright and General Chang were both influenced by telepaths under Romulan control. This suggests that Nanclus had a more active role than he appeared to in the film.
In the novel Cast No Shadow, "Nanclus" is said to be a Tal Shiar operative who used that name and the role of an ambassador as a cover. When the conspiracy was uncovered, "Nanclus" was recalled to Romulus, where the official report was that he was executed by disintegration. In reality, though "Nanclus" was given a different identity and assignment. In 2300, he was involved in a failed attempt to support Kriosian terrorists in rebelling against the Klingon Empire. After the plan failed, "Nanclus" and the other Tal Shiar chiefs were killed by a Klingon agent named Kaj.