From Star Trek: Theurgy Wiki
The Obsidian Order was formed in the 19th century, under the theoretical authority of the civilian Detapa Council. However, in practice it and the Central Command operated with complete autonomy. The Obsidian Order was responsible for matters of security and espionage, and was therefore forbidden from interfering in military operations. The Obsidian Order and the Central Command frequently came in conflict with each other, particularly since members of the Cardassian military were not immune to Obsidian Order inquiries. The only restriction that the Order had was that it was explicitly forbidden to possess military equipment of any kind.
The organization of the Obsidian Order was separated into semi-autonomous cells, which were insulated from each other so that the capture of any particular operative has a limited ability to compromise the organization. Certain agents of the Order were also equipped with cranial implants so that they could better resist torture. They also developed a substance called promazine, which its operatives used in case of capture.
In 2371, the Obsidian Order was responsible for the abduction and surgical alteration of Kira Nerys in an operation to expose Legate Tekeny Ghemor as a sympathizer to the Cardassian dissident movement. The Order also sabotaged the USS Defiant's attempt to vaporize a silithium-laden comet headed for the Bajoran wormhole, hoping to undermine the peace treaty between Cardassia and Bajor.
Also that year, the Obsidian Order was engaged in a major operation to mount a joint attack with the Tal Shiar on the Founders' homeworld. Masterminded by former Order head Enabran Tain, and in blatant violation of the non-military restriction, the Order had assembled a fleet of warships in the Orias system, consisting of an advanced version of the standard Galor-class starship, the Keldon-class, without the knowledge of the Central Command, and thereby de facto becoming an illegal military organization. The attack, however failed due to a Changeling infiltrator, and the entire fleet was destroyed in an ambush by the Jem'Hadar. This failure crippled the capabilities of the Obsidian Order, essentially causing it to cease to exist and ending its short-lived existence as a military organization.
With the looming threat of the Dominion, and without the Order's tight control over popular dissent, a civilian uprising overthrew the Central Command in 2372, reinstating the authority of the Detapa Council. After the Cardassian Union joined the Dominion, the Obsidian Order's role was assumed by Dominion security and the Cardassian Intelligence Bureau, who by all accounts were just as efficient.