An extreme form of Quickening; largely believed to be only myth, the Dark Quickening, generally disastrous and often permanent, can change the receiving Immortal into a savage, remorseless killer.
In the grip of a Dark Quickening, Duncan threatens Methos despite being on holy ground
In the Big Finish productions, it was indicated the victim might have his entire personality suppressed, and replaced by the darkness of the powerful evil Immortal, as happened to Caspian. (See: The Big Finish Audio Highlander The Pain Eater).
Dark Quickenings may be all but impossible to reverse; only one instances where it was achieved can be cited from the series. Two others discussed in 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 The Kurgan. Connor eventually regained control of himself, suppressing the violent tendencies of the Kurgan, with the aid of his clansman, Duncan MacLeod. (Highlander: Dark Quickening) The other story was one in which Sunda Kastagir asked the MacLeods to stand watch while he closed himself off in a mine and fought the contamination.
In the series, Duncan MacLeod was forced to take the head of Coltec, after he was corrupted by a Dark Quickening when he took the head of the evil immortal, Kant in the episode, Something Wicked. When they first met, Coltec had purged Duncan of the rage and madness that would have come to rule him, taking it into himself. When Duncan took Coltec's head, he got back his own darkness as well, and had no defense against it, having never mastered himself.
Under the influence of the Dark Quickening, Duncan took the head of the unfortunate Sean Burns, a great and good man. Methos managed to help Duncan get control of himself long enough to take him to a long lost Holy Spring. There, MacLeod went into the water and managed to defeat his evil self in the domain of his own mind, with the encouragement of Sean Burns either imagined or real, in the episode, Deliverance.