Throughout the history of Immortals, the Quickening, the power taken by one Immortal upon beheading another, has been mysterious even to them. In this transfer, battle prowess, genius in the arts, and even love affairs have been transmitted to the recipient. But one of the most dramatic transfers comes from a simple yet even harder to define concept : Good And Evil. In most cases, if the Immortals involved are neither extraordinarily good nor incredibly evil, the recipient more or less keeps themselves as they already are. At least in one known case, the barbarian leader Darius abandoned his conquest of Europe after beheading a revered Holy Man immortal at the entrance to Paris, and instead became a simple priest, the whole of his existence now devoted to peace. But the inverse situation is well known enough to the community of Watchers that protocols exist to abandon their non-interference oath and put down Immortals known to have gone mad by this process. When Duncan Macleod first introduced Richie Ryan to Darius and told him his story, Richie asked if a good Immortal could go bad in that way, and Duncan's hesitancy in answering spoke volumes.
But this process can occur, not only by the taking of one extraordinary individual, but by accumulation over time. The problem is, this tends to be only for the evil side of the equation, since those who would and do seek to take the heads of good Immortals tend to be irredeemable types themselves, much less apt to be affected by those they take. Therefore, it takes a miracle to turn an evil Immortal good by this process, but sheer weight of time and the types of opponents one faces makes this a very possible outcome for good ones - in this case meaning those who keep to the Rules and choose to not seek out others for the sake of power, living and enjoying their lives.
Within the series' timeline, it is first spoken of in 1958, when Duncan Macleod is spared a battle with a psychotic Immortal by his old friend Coltec, a Native American shaman and Hayoka, one whose task it is to take in the evil of the world, similar to the Buddhist concept of Bodhisattva. Coltec then declares he must withdraw entirely from The Game until he has mastered the great madness he just absorbed.
In a retcon of the movies, Connor Macleod's battle with The Kurgan was no longer the final deciding battle in Immortal history, merely a very important one. Later adventures, both screen and print, showed that Connor now struggled with having the Kurgan's darkness inside him, almost losing both his mind and his identity to the man who killed his mentor and raped his wife. This struggle was later cited as part of the reason he pulled back from the Game.
In 1996, Duncan Macleod again encountered Coltec, who had selflessly but foolishly continued to absorb evil, until he could take no more and underwent a Dark Quickening after dealing with a sociopathic petty thief. The Hayoka had always warned of this, but Macleod, who had been saved from a crippling rage after the slaughter of his adopted Native American family by Coltec's actions, hesitated until the rage-drive fallen Shaman left him no choice.
Feeling overwhelmed by not only Coltec's madness but by the many evil or crazed opponents he'd bested over the centuries, Duncan now lapsed into madness himself, not only attempting to kill Richie Ryan (now an Immortal as well) but scaring him into hiding when only rescue by Joe Dawson could save him. Dawson knew his path was clear, and that, however much he admired him, Duncan would have to be put down, likely by he himself, so that the madness could pass no further. When a psychiatrist, a good man Duncan had known since World War One, tried to approach him to help, Duncan sank as low as any of his opponents ever had and lured the man in by feigning interest in what he had to offer, killing him without hesitation. During this time, he also victimized an innocent sailor and his wife, all but destroying their lives. But Methos bid Joe Dawson let him try and help Macleod.
Duncan was actually somewhat regretful for his actions, but no longer saw a way back. Even with the desire to try and find one, he actually considered taking Methos head on holy ground. Methos knew of a holy spring, and while not a cure, it would permit Duncan to confront his evil side. With the spirit of the friend he slew forgiving him, Duncan bested the dark being he was becoming and was cleansed. Methos never said how he came to know of this spring, but it is indicated he himself once used it, possibly indicating also that his dark times could have been the result of the Dark Quickening.
Presumably, facing this struggle aided Duncan in overcoming the millenial demon Ahriman, as well as in taking in the Quickening of Jacob Kell, though in this he had the direct aid of the spirit of his kinsman, Connor.