The Wrath were a militaristic species. They were Igris that were unified by the force of Tenebris' mind.
They fought for several years against the Drashani Empire and won. Afterwards, they went to war against the Earth Empire.
The Wrath had no individual free-will. They put the minds of their prisoners into a machine which functioned as a surrogate for this absence. Their spacecraft left an antimatter trail.
The Doctor encountered the Wrath in his sixth and seventh incarnations. (AUDIO: The Acheron Pulse, AUDIO: The Shadow Heart)