Mar Loc

Mar Loc was a male Sindareen from the planet Sindar. In 2408 he was employed as a scientist on the Forever World when he used the Guardian of Forever to travel back in time to 2368.

Going by the name Eza and working as an aide to Ambassador Nici, he was able to attend a peace conference on the Enterprise-D and slip raxatoxin into Counselor Deanna Troi's drink killing her later that night. In 2408 of the alternate timeline created by Troi's death, Admiral William T. Riker uses the Guardian to travel back and prevent Troi's death thus restoring the original timeline. (TNG novel: Imzadi)