Deanna Troi masquerading as Major Rakal
Major Rakal was an operative in the Romulan Tal Shiar.
In 2368, she was killed by members of the Romulan underground movement, so that Deanna Troi could be surgically altered to take her place. As Rakal, Troi could then take part in a covert mission to allow the defection of Proconsul M'ret and his aides to the Federation. (TNG: "Face of the Enemy")