You may be looking for the fictional Professor James Moriarty or his holographic counterpart.

Moriarty is the last name of the actor who appeared as a dead USS Brattain crewmember in the Star Trek: The Next Generation fourth season episode "Night Terrors" in 1991. The same year, he played a dead Klingon officer in the fifth season episode "Redemption II". He also appeared as a different Klingon in this episode in several scenes in the Klingon nightclub and the Great Hall.