The Shamarians were a post-Dominion War Cardassian faction that vied for control of the Cardassian Union in the aftermath of that conflict. The faction was led by Jagul Koros Shamar.
They sought to establish Shamar as the undisputed leader of a new Cardassian Empire. He was backed by many of his former officers and their starships. (The Dominion War Sourcebook: The Fires of Armageddon)