The Obsidian Order reference article from the English Wikipedia on 24-Apr-2004
(provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Obsidian Order

Have you considered sponsoring a child
The Obsidian Order is a Cardassian secret police organization in the fictional Star Trek universe.

Security Chief Odo of Deep Space Nine remarked that it was one of the most brutally efficent organizations in the galaxy. The Order kept very close tabs on all Cardassian citizens to ensure loyalty, and was greatly feared. It was said that the average Cardassian could not sit down to dinner without the contents of the said meal being noted and logged by the Order. Odo also noted that the Order caused people to disappear for even less than eating something the order did not approve of.

Elim Garak was a member of the Order, before being exiled from Cardassia to Deep Space Nine by his father, Enabran Tain. Tain had retired for a time, the only director to ever do so. Garak became an ally of the Federation who used his knowledge to aid them in the war against the Dominion.

The Obsidian Order and the Tal Shiar had allied in an attempt to destroy the the Dominion. This attempt was not sucessful, and it led to the downfall of the Obsidian Order.