It's shown in the episode "The Mind's Eye" that Klingon Ambassador Kell was a traitor in cahoots with the Romulan Empire.
Is there anything in the EU which speaks to what happened to the ambassador? He was to be searched by the Klingons for an E-band transmitter. Presumably, Kell was executed, though does anything state this absolutely? For all we know, an Organian saved his life at the last instant. Doubtful, but it is possible.
There are no further mentions of Ambassador Kell in any of the TV shows or films, nor is there anything within the show script that says what happened to him after leaving the Enterprise.
Given that he was accused of being a traitor to the Empire, we can reasonably assume that he was either tried and executed for his treachery or simply killed by Vagh or one of his men as soon as the transmitter was found.
The final stage direction pretty much gives it away:
Kell takes a deep breath, knows he's finished. Picard nods to Vagh.
There is no canon where this question is answered definitively; however, it's quite likely Kell, like most traitors, was executed.
It should be noted that there is a possibility that he has sent to Rura Penthe (Klingon prison planet from Star Trek 6). Given the fact that he was working with the Romulans, this is unlikely. The Romulans are considered honorless by most Klingons so working with them would be likely be punished by death.
