Quickening (Highlander)

The Quickening is a phenomenon in the Highlander films and television series. Beheading a character known as an "Immortal" produces a powerful energy release from their body called a "Quickening."

The lead cast member of the television series, Adrian Paul, explained, "The Quickening is the receiving of all the power and knowledge another immortal has obtained throughout his/her life. It is like the receiving of a sacrament or a massive orgasm." The series producers stated, "The power of the Quickening is the equivalent to a major electrical storm hitting—windows explode, lights short circuit, it is almost as if the victorious Immortal is in the center of a lightning storm."

In the Highlander universe, the energy released by a Quickening is absorbed by the Immortal responsible for the beheading. The series producers explained that if "an Immortal is decapitated by something other than the sword of the Immortal he was fighting, (...) what we thought was, as long as an Immortal is present, he gets the Quickening."
If an Immortal is beheaded and there is no Immortal nearby to receive the Quickening (if, for example, the beheader is a mortal), then the Quickening dissipates without effect.. The series producers have also revealed that, "if there is no Immortal present, then the Quickening just goes to the Source," but it is not known yet what the "Source" exactly is.

On rare occasions, the Quickening overwhelms the personality of the Immortal, even turning a good Immortal evil. This occurrence is known as a Dark Quickening in the series version. The converse can also happen; Darius is the only known example of a Light Quickening.

The Immortal characters are able to sense a nearby Quickening, as well as an ability to know which Immortal has been killed. Such abilities are demonstrated in Highlander: The Series, when Duncan MacLeod falls on his knees following the beheading of his friend, Lucas Desiree. Desiree is beheaded by Howard Crowley, and MacLeod knows that his friend has died.

The Quickenings in the Highlander movies vary greatly, as the creators were still experimenting with the concept.

In the original Highlander film, when an Immortal beheads another Immortal, the survivor is overwhelmed by an invisible force embodied in a bright flashing light, at which point the dead Immortal's body levitates, releasing an energy charge in the surrounding area.

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Wikipedia