The Quickening refers to a phenomenon in the Highlander films and television series. When an Immortal is beheaded, there is a powerful energy release from their body which is called a Quickening. Lead Highlander: The Series actor Adrian Paul explains, "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 producers describe it so: "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."
This energy is absorbed by the Immortal who did the beheading. Panzer explains 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, for example if the beheader is a mortal, then the Quickening dissipates in the sky. Panzer says, "If there is no Immortal present, then the Quickening just goes to the Source." It is not known yet what the Source exactly is.
When a good Immortal beheads an evil one, it rarely happens that the evil Quickening completely overwhelms the personality of the good Immortal, making him evil. This is a Dark Quickening. The contrary can also happen; Darius is the only known example of a Light Quickening.
An Immortal can sense when a Quickening takes place nearby and he knows which Immortal is dead, as demonstrated by Duncan MacLeod in Highlander: The Series. He falls on his knees when his friend Lucas Desiree is beheaded by Howard Crowley, and he knows it is Lucas who died.