This Merlin on BBC photo might contain
hunk, six pack, abs, ab crack, flat stomach, skin, skintone, nude colored, partial nakedness, and implied nudity.
STORY: The song is about King Arthur's son Mordred, who was born between Arthur and his half-sister, Morgaine Le Fay. It's Morgaine singing to him; persuading him to kill his father, which he ends up doing.
Basically that is the story