The Sindareen were a humanoid species native to the planet Sindar. They spoke with a hissing sound because of nictitating membranes on their necks. Their natural aggression allows them to block telepathy. However, after significant contact with an individual, the telepath can get a clear reading. In 2368, Deanna Troi was able to learn that the Sindareen delegation was not entirely forthright about their supposedly peaceful intentions. The Federation pulled out of peace negotiations and the Sindareen economy soon collapsed. With Federation help they were eventually able to rebuild, but never regained the height of their power. (TNG novel: Imzadi).
Still smarting over these events in 2374, a Sindareen information dealer named Tor Vot stumbled across an opportunity to raise his people back to their former prominence. A cohort of Future Guy, who was attempting to alter the outcome of the Dominion War in his favor struck up a deal with him. The humanoid from the 29th century provided Tor Vot access to new sources of wealth as a reward for providing the Dominion with Federation Alliance fleet deployments. Such information included tactical data indicating that Betazed was vulnerable to an attack. Tor Vot was then promised access to cormaline deposits on Makar VI if he could deliver USS Lambda Paz captain Limis Vircona to his benefactor from the future. Cardassian Gul Enic Hadar, seeking to avenge his brothers death, had hired Tor Vot to do the same. This attempt was a failure thanks to intervention from 31st century temporal agent Jonas Grabowski.
Sindareen battle cruisers apparently had advanced knowledge of 24th century Federation starship design, which allowed two of their ships to outmaneuver the Lambda Paz. They were defeated when Commander Ronnie Kozar used an unexpected tactic. (Star Trek: Lambda Paz: " The Tides of War, Part I")