A Klingon who arrives at Deep Space 9 in 2371 posing as an ordinary warrior on shore leave, in reality an intelligence agent sent to spy on a visiting Romulan delegation.
Bo'rak is a Klingon who appears in the third-season Deep Space Nine episode Visionary. He and two companions, Morka and Atul, come to the station while their ship is under repair, at the same moment that Romulan officials are aboard to be briefed about the Dominion.
They drink, pick a fight in Quark's bar and are locked up by Odo, which looks like ordinary Klingon rowdiness. O'Brien, thrown a few hours forward by a radiation accident, witnesses events that suggest otherwise, including his own death from a booby trap.
Odo's investigation shows that the three belong to Klingon Intelligence and had hidden a surveillance device in the wall of the Romulans' quarters. They are expelled from the station.