The elite Klingon bodyguard known as the Brotherhood of the Sword, sworn to protect Chancellor Gowron, whose presence at Ty'Gokor complicated Sisko's mission to unmask a Changeling in 2373.
The Yan-Isleth, a name translated as the Brotherhood of the Sword, are the personal guard of the Klingon Chancellor. They are chosen for skill and loyalty, and stay close to him at public ceremonies.
They feature in the Deep Space Nine episode Apocalypse Rising. Sisko, Worf, Odo and O'Brien, surgically disguised as Klingons, infiltrate the military headquarters at Ty'Gokor during an induction into the Order of the Bat'leth, intending to expose Gowron as a Changeling with special emitters.
Getting near the Chancellor means getting past the Yan-Isleth, and the team is seized before it can act. The supposed General Martok then urges Sisko to kill Gowron in defiance of honour, which leads Odo to realise that Martok, not Gowron, is the impostor.