*medium spoiler level*
In The Wheel of Time
, a female Forsaken
Lanfear, also known as Mierin Eronaile, was an Aes Sedai researching One Power in Age of Legends. Upon a search for Power that wouldn't be limited by division to genders (saidar/saidin), she stumbled into Dark One's prison, drilled a hole (known as Bore) into it and released Dark One, effectively starting War of Power, or, War of the Shadow, which led into Breaking of the World and end of Age of Legends.
Although it is believed that she didn't realize what she was doing when she freed Dark One, she DID go over to him (?), becoming one of the dreaded Forsaken, or Chosen as they call themselves.
Lanfear was extremely beautiful woman, maybe even most beautiful woman of her times, and also very ambitious. She was said to love Lews Therin Telamon, although LTT's own opinion was "You never loved me, Mierin! You loved only power!" (notice the minorcase 'p', he didn't mean Power). She is also very strong in Power, most seem to think she was next only to Ishamael among the Forsaken, and some even whisper she might have been the strongest, only hiding it. She also liked Tel'aran'rhiod much and claimed it as her territory, even though she wasn't as apt there as Moghedien was.
Some think that Lanfear is potential candidate for redeeming and coming back to Light. For one, she is believed to have entered Shadow only for the power; if Light gave her more power, she would come back. Also, she has proven she isn't very loyal to Shadow in any case: She has revealed her plans about usurping Dark One and becoming (demi-)God herself, along with Dragon Reborn. She also helped Rand al'Thor, being only Forsaken not basically attacking Rand at sight.