Quickening
The literal opposite of a Dark Quickening, it has been called a Light Quickening.
In the TV series, the immortal Darius was rumored to have changed after beheading another immortal, a holy man at the gates of Paris. He was named Emrys in the Highlander novel, Highlander: Shadow of Obsession. He chose to face Darius alone and lost. After the Quickening, Darius walked away from his army and took up residence on holy ground.
The Big Finish audio production suggested that Immortals might be gradually influenced by the Quickenings they take, thus it making it possible for an evil Immortal to take so many good and righteous immortals that his internal balance eventually tips and he becomes a far less evil person, even a perhaps a good person.
Trivia[]
While Duncan told Richie the story of the "Light Quickening" of Darius in Band Of Brothers the then pre-immortal theorized that if a good immortal beheads an evil one, that could change the good one into an evil person, a first hint of the "Dark Quickening."