A Quickening in an Immortals life force, it holds all their strength, knowledge and power. The Quickening can only be released from the body after decapitation, once this happens the Quickening is absorbed into the victor, or the nearest Immortal if the decapitation happened by accident or by mortal means.
Quickening’s are massively destructive, though the extent of the devastation is dependent on how old and how powerful the immortal is. During the episode “Comes a Horseman”, Duncan MacLeod and Methos received a shared Quickening that almost completely destroyed a factory from two characters that were over 3000 years old, but during Endgame, Duncan MacLeod received a Quickening so powerful it cleared the atmosphere of clouds and destroyed a major power plant in New York, this Quickening was from Jacob Kell who was just over 600 years old.
If an Immortal received too many Quickening’s from ‘evil’ Immortals, then the build up can eventually overcome the immortals psyche and turn them into a dark, twisted version of themselves. This happened to Duncan MacLeod in season 4 of the television when an Immortal Native American named Coltec was eventually overwhelmed by the Dark Quickening. Duncan MacLeod eventually killed Coltec and received the Dark Quickening himself, he was eventually saved by Methos, though the cost was great, as during his ‘Dark Time’ Duncan had killed one of his oldest friends, Dean Burns.
As with the Dark Quickening, the same can happen in reverse to an ‘evil’ immortal. Darius was an ancient immortal warlord who roamed the world searching for Immortals, when he came to Paris he was faced with an Immortal monk who protected Paris. Darius slew the immortal and received his Quickening, the pure, Light Quickening overcame Darius’ psyche and since then he dedicated his life to the church and spent the next thousand years on holy ground.
Holy Ground Quickenings
One of the three rules for Immortals, there must be no fighting on Holy Ground. This rule has been broken twice in Highlander cannon, both times with disastrous consequences.
In 79AD two immortals fought in a temple on Pompeii, when the final stroke fell the volcano erupted from the Quickening, killing the immortal victor as well. In 1993, Connor Macleod faced Kane in a Buddhist shrine, when Kane went to make the killing blow Connor blocked him with his Katana and went to finish the fight, Connor’s sword shattered to dust before making the blow. Rather than attack a defenceless man on holy ground, Kane wisely chose to save the fight for later.