The IRW Sithesh was a D'deridex-class warbird in service in the early 25th century. It was affiliated with a Reman faction loyal to former Praetor Taris and commanded by Commander Karatek. (STO - Romulan Mystery mission: "Ground Zero")
In 2409, the Sithesh welcomed a Federation starship in the Hobus system. Commander Karatek threatened that any deviation of the visitor would be met by force. After the Starfleet vessel scanned the remains of Hobus III, the Sithesh attacked it but cloaked after taking damage. The Sithesh engaged the Starfleet ship again at Hobus I but was forced into retreat once more. An away team discovered and sabotaged the Reman decalithium mining operation on the Hobus I planetoid and found prove for Remans' culpability in the destruction of Romulus and Remus in 2387 by causing the Hobus supernova.
The Sithesh, now accompanied by two squadrons of T'varo-class light warbirds, engaged the Starfleet vessel a final time. Starfleet destroyed all attacking Reman vessels and reported its findings to Admiral T'nae. (STO - Romulan Mystery mission: "Ground Zero")