A Sentinel, in Ardanan society, was an individual charged with security matters in the Cloud City of Stratos (TOS episode: "The Cloud Minders"), as well as being responsible for running and maintaining the physical workings of the city. Sentinels were provided with the knowledge to operate the city's anti-gravity engines and other functions by the introduction of a bio-engineered parasite, which created new neural connections within a host's brain.
In 2377, Ardana's High Advisor, Larin Nelois, apparently plotted to have one of these parasites, held in a small carn, infect a member of the crew of the USS da Vinci in order to, in essence, create a new Sentinel, who could then affect vital repairs to the city. While the victim of this plot, Crewman Fabian Stevens, was able to gain much of the Sentinels' knowledge, the parasite nearly killed him, and he and his fellow teammates were unable to prevent Stratos from crashing. (CoE eBook: Signs from Heaven)