A Satarran, as he originally appeared to the crew of the Enterprise-D
The deception is revealed by a phaser blast
Commander Keiran MacDuff was an alias assumed by a Satarran who posed as the executive officer on board the USS Enterprise-D.
After altering the memories of the Enterprise crew and changing the computer records in 2368, MacDuff convinced the crew that they were at war with the Lysians, and urged the destruction of their mission objective, the Lysian Central Command. The Satarrans were at war with the Lysians for decades but could not destroy the facility because their weaponry was similar to the Lysians and Central Command was heavily guarded (by their standards).
After the Enterprise encountered, and easily destroyed, a Lysian destroyer ship, Captain Jean-Luc Picard began to express doubts about their mission. MacDuff attempted to assuage these doubts, while he privately approached Lt. Worf and confided that he had doubts about Picard's willingness to perform their duty, and that MacDuff would count on Worf's support if it became necessary to assume command of the ship.
Though the Enterprise came within weapons range of the Central Command, the ease with which the Galaxy class starship had penetrated the Lysians' defense perimeter increased Picard's doubts, and he questioned if they were really at war with the Lysians; after all, how could a species centuries behind them in weaponry be a serious threat?
MacDuff's mission failed when, on realizing that the Central Command itself was practically defenseless against the Enterprise, Picard refused to fire. MacDuff attempted to assume command and ordered Worf to open fire. Worf likewise refused. MacDuff tried to overpower Worf and fire himself, but was hit by a phaser beam by Commander Riker, revealing his true identity. Together, Riker and Worf were able to incapacitate MacDuff and prevent him from destroying the station.
MacDuff's faked identity states that he was born on September 27th, 2334, as son of Joseph and Les MacDuff on Gamma Canaris N. He entered Starfleet Academy in 2352 and graduated in 2356. (TNG: "Conundrum")