Klingon Intelligence was the intelligence gathering agency of the Klingon Empire responsible for conducting espionage and covert activities. Agents who fail to accomplish their mission are dishonored and frequently exiled from the Empire. (DS9: "Visionary", "Trials and Tribble-ations")
Arne Darvin was an agent in Klingon Intelligence, surgically altered to appear Human. He was assigned in 2268 to poison a shipment of quadrotriticale bound for Sherman's Planet. Darvin's identity was discovered when a tribble reacted to him. (TOS: "The Trouble with Tribbles"; DS9: "Trials and Tribble-ations")
In 2371, Klingon Intelligence sent agents Bo'rak, Atul, and Morka to Deep Space 9 to observe a Romulan delegation being briefed by Starfleet concerning the Dominion. They planted a surveillance device near the briefing room, by modifying a replicator into a transporter. Their identities were exposed after Miles O'Brien witnessed himself being killed by the surveillance device's booby-trap mechanism in the future. (DS9: "Visionary")
In 2372, Klingon Intelligence had discovered that the Cardassian planet Loval was the site of a secret weapons research facility, which led a Klingon Bird-of-Prey to attack the planet. (DS9: "Return to Grace")