First Appearance: Fantastic Four #94 (January 1970).
Appearances: Too many to list.
Years Active: 18,000 B.C.E?-? (see below)
Agatha Harkness is a leader of the witches of New Salem. Until recently Agatha was thought to be just an aged woman who had been a member of the witches of Salem (Mass.) up through the Salem Witch trials, at which time Agatha led the witches west and helped establish their new stronghold in the Rockies.
However, Silver Surfer #135 revealed that Agatha is at least 20,000 years old. Her origins remain unrevealed, but what is known is that as a young blonde she wandered into a community of centuries-old witches at that time and joined them. In an issue of New Warriors it was revealed that in the past Agatha was involved in turning over various sorcerers to the witch-killing authorities, on the grounds that she was strengthening the witch community by ridding it of those who could not defend themselves. And in X-Men: Hellfire Club #1 someone named "Abby Harkness," who had the blonde hair of a young Agatha, was shown fleeing Salem on horseback with the son of Hiram Shaw.
Note: I shouldn't be surprised at the continuity error in Hellfire Club, should I?