The Legion of Assassins, also known as the League of Assassins, was an organization within the Romulan Star Empire.
The organization treated their assassins extremely harshly for revealing any secrets. Those that were guilty of such a crime faced punishment a number of ways which, including using the accused as target practice, performing medical experiments or simply harvesting their organs, which they did to most condemned prisoners. They were reported to have been trained to resist mind melds.
By 2377, they had sent a Jekrit along with another assassin in order to kill Romulan dissident Regimol. However, they were apprehended where they were interrogated by Admiral Alynna Nechayev during the time of the Genesis Wave crisis. (TNG novel: The Genesis Wave, Book 3)