The Tal Diann was the internal affairs division of the Romulan Star Navy of the Romulan Star Empire. The Tal Diann reports to the Star Command and serves as a form of naval intelligence separate from the Tal Shiar intelligence service.
The organization was created by the Imperial Fleet as a result of the growing power of the Tal Shiar. This is because of the fleet's belief that trusting the intelligence services good graces would have been unwise and thus the Tal Diann polices the fleet. They serve as the counter intelligence wing against Tal Shiar operations who subjected all other organizations, including the civilians, to intense scrutiny.
The head of the Tal Diann is known as the Chief of Military Intelligence who holds the rank of Admiral within the Imperial Fleet. The CMI sits on the High Tribunal, something that has been an annoyance of the Tal Shiar. The Chief of Military Intelligence also reports directly to the Praetor. Junior positions include Chief of Planetary Forces Intelligence, Chief of Stellar Forces Intelligence, and Chief of Operations. (LUG RPG: The Way of D'era: The Romulan Star Empire)
The most visible members of the Tal Diann were the legates or belisaro. Each ship is assigned a belisaro who serve as the Star Command's eyes and ears. During times of war belisaros take command of the unified Star Navy so to maintain the political balance. They serve in the Praetor's name and have ultimate authority in matters such as promotions, troop movements, strategies and other war duties. (The Dominion War Sourcebook: The Fires of Armageddon)
In 2268, Praetor Kanetar issued more political power to the Tal Diann to serve as a safeguard against the rising power of the Tal Shiar. They were tasked with monitoring relations between the Star Command and the senatorial fleets.
A training camp for the organization is located on Coravus Prime where naval recruits are sent to receive better training for intelligence operations. (LUG RPG: The Way of D'era: The Romulan Star Empire)
An organization by the same name, known as Romulan Military Intelligence or Republic Intelligence in Federation Standard, operated as the Romulan Republic's primary intelligence service in the 25th century. They were a direct counterpart to Starfleet Intelligence. (Star Trek Online; Bait and Switch: Saith Daepahr hrrafv Llaiirevha glossary)
In 2410 the Tal'Diann provided bodyguards for a group of Republic senators sent to serve as a diplomatic delegation at a peace summit with the Star Empire. (Bait and Switch: "Saith Daepahr hrrafv Llaiirevha, Part I")