...it takes a Grey Warden to slay an Archdemon, the leader of the Darkspawn. If anyone else does, the soul of the Archdemon will travel into another Darkspawn and create another Archdemon in its place. [she's not going to mention that in all cases but hers, the soul tried to attach itself to the Grey Warden who slew it, but both souls were obliterated in doing so]