In "Highlander: The Series", it is said that the immortal who survives a duel with another will not only absorb power but also some physical skills and can temporarily experience personality traits and flashes of memory from the one beheaded. In rare cases, an immortal can be overwhelmed by the personality of the person whose Quickening they just absorbed, leading to a shift in their own personality, such as in the season 4 episode "Something Wicked." In the same episode, Duncan MacLeod reveals some immortals speak of this phenomenon as a "Dark Quickening," when the personality is overloaded and corrupted.