The Sindareen are a humanoid species native to the planet Sindar. Physically stronger and more resiliant than humans, they are barely affected by a phaser blast set on "stun" and are merely stunned by a "kill" setting. When they speak, sound issues predominantly through nictating membranes on their necks.
Their natural aggression makes it difficult for empaths to read them; however, after a significant time with the same individual, Deanna Troi was able to penetrate this resistance and get a clear reading. In 2359 she was able to realize that Mar Loc was ready to kill her, and in 2368 she was able to detect that the Sindareen delegation was attempting to deceive other conference attendees about their supposedly peaceful intentions.
As a result of this deception, 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 powers.
In 2408, a Sindareen scientist named Mar Loc, (using the disguise of an Orion female), used the Guardian of Forever to travel back to the peace conference and kill Troi before she could reveal her deception. This created an alternate timeline in which the Sindareen became much more powerful. However, this alteration was undone when Admiral William T. Riker traveled to the past and saved Troi's life.