Admiral Dagarth was a Romulan military officer during the Earth-Romulan War.
In 2156, Dagarth's ship, the ChR Nel Trenco, responded to a Haakonan attack on Trilakis and was not heard from afterward. Dargarth and her crew were presumed to be dead. (ENT - The Romulan War novel: Beneath the Raptor's Wing)
In 2157, Dagarth and the Nel Trenco were back in action. Dagarth had a mutagenic virus deployed against Haakona Prime, transforming the entire civilization into a different species. Admiral Valdore, outraged that Dagarth would dare undertake such an action without authorization, killed Dagarth with his Honor Blade. (ENT - The Romulan War novel: To Brave the Storm)