Dark Quickenings may be all but impossible to reverse; only two instances where it was achieved can be cited. The first from the comic books, the canonical status of which are questionable, in which Connor MacLeod was alleged to have suffered a Dark Quickening after he took the head of his ancient enemy, The Kurgan. Connor eventually regained his personality, suppressing the violent tendencies of the Kurgan, with the aid of his clansman, Duncan MacLeod. (Highlander: Dark Quickening)
Under the influence of the Dark, Duncan took the head of the unfortunate, Sean Burns, a great and good man. With Methos' help, MacLeod managed to purge the evil within him by performing a ritual of self-cleansing, in which he battled his evil self in the domain of his own mind in the episode, Deliverance.