Kieran MacDuff is the main antagonist of the Star Trek: The Next Generation episode "Conundrum".
He was portrayed by Erich Anderson.
Kieran MacDuff was the name used by an agent of the Satarrans, a race at war with the Lysians. Both races were evenly matched in terms of technology, but inferior to the Federation. MacDuff thus enacted a plan to trick the Federation into ending the war for them. He approached the Enterprise in a small ship and used a weapon to give the entire crew amnesia, leaving them with the knowledge to operate the ship but no personal information. He then altered the ship's records to make them believe the Federation was at war with the Lysians.
Disguised as a human Starfleet commander, MacDuff boarded the ship, playing the part of another amnesia-stricken officer. When the crew retrieved the altered files, they stated that MacDuff was the ship's first officer, which the crew accepted without question. They were also informed that the Enterprise was on a mission to destroy the Lysian central command and were to maintain communicator silence while other elements of Starfleet waited to act.
The Enterprise entered Lysian space and were quickly intercepted by a Lysian ship, which attempted to communicate. MacDuff convinced Picard to ignore the hails, suggesting the Lysians might erase their minds again if they did. When the Lysian ship fired on them, the Enterprise destroyed it. Picard told MacDuff he was feeling uneasy about the idea of killing people he didn't know for reasons he didn't understand. MacDuff approached Worf, suggesting they were the two warriors put on board the ship as security and should be ready to act if the others faltered.
The Enterprise approached Lysian central command but Picard continued to be confused by the Lysians' inferior technology and refused to give the order to fire on defenceless people, instead trying to open communications. MacDuff attempted to take command but Worf refused to support him. When MacDuff then attempted to fire the ship's weapons by force, both Riker and Worf hit him with sustained phaser blasts, revealing his alien nature and probably killing him.