The literal opposite of a Dark Quickening. A light Quickening occurs when an evil immortal has killed so many good and righteous immortals that his/her internal balance hits critical mass and he/she becomes a good or at least far less evil person.
There have been two Light Quickenings though one is unconfirmed. The Kurgan was temporarily resurrected in Kurgan Rising, a resurrected Connor MacLeod allowed him to take his head Duncan MacLeod killed the Kurgan while he was recovering.
The ancient immortal Darius was rumored / believed to have absorbed a Light Quickening after beheading another immortal, Emrys, who chose to face Darius and his army alone. After the Quickening, Darius walked away from his army and took up residence on holy ground.