The Sidhe elder was a Sidhe who plotted to create a Sidhe queen.
A short time after the birth of Lord Godwyn's daughter, Princess Elena, the elder came and cast a spell on her. He created a Sidhe inside her who would completely possess her when she married Arthur Pendragon. He had his Pixie servant, Grunhilda become her nurse and sprinkle Sidhe dust on her every night as she slept. When she was 20, Uther Pendragon and Godwyn, who were old friends, arranged a marriage between her and Arthur. Arthur's servant, Merlin discovered the plot. Grunhilda informed the elder of this and he tried to kill Merlin in his sleep, however, he was killed by Merlin's Sidhe staff which he previously acquired from another Sidhe. Merlin later foiled his plan and killed both Grunhilda and the Sidhe inside Elena, Elena and Arthur then agreed they didn't love eachother and cancelled the marriage. (The Changeling)