What is the Quickening? The official definition from the producers is "An Immortal can only be killed by beheading.
When an Immortal is killed, the Immortal who killed him receives his power and his essence and his knowledge through a mystical process known as "The Quickening."
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.
" Traditionally 'quickening' is a term used to describe the first discernible movement of a baby in the womb, i.e. when the fetus first shows signs of life.
It has been theorized that Greg Widen chose this term do describe the peculiar life force of Immortals.
As Adrian Paul describes it, "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. That's my motivation."