The Ten were a powerful cabal that existed within the Romulan Star Empire.

This group consisted of a secret alliance formed by various powerful Romulans who were fed up by their empire's arrogance and disregard. Each of its members were a shrouded figure who conveyed in secret with their combined power being quite great as they commanded ships as well as troops, held private funds for weapons along with contracts with the Klingon Empire. They had only made a single mistake which had brought an incident of government investigation to them but beyond that, their actions had produced results. Its aim was overthrowing the government.

The members of the Ten had no names, no faces and no unfiltered voices which betrayed their identities. Instead, their secrets went deep and their plans were whispered in their secret meetings. They were called out by number with origins of the organization and its leadership falling to an individual called the First. It was the First who knew all the members of the Ten by their name. This cabal did not tolerate traitors within their midst and executed them for any such actions. The cause of the organization involved the use of the Grand Design as part of their plans to gain power.

By 2279, Praetor Tahn was a member of the Ten. (TOS novel: The Pandora Principle)

With Tahn being labelled a member of the Ten, it seems somewhat odd that he would seek to overthrow his own government when he was its effective leader. However, its possible that the use of the term Praetor might be more in line with Rihannsu novel continuity which may mean that Tahn was simply a high ranking as well as influential member of the government rather than its leader.
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