During Chris Claremont's first run as X-Men writer, agents of the Hellfire Club kidnapped the X-Men and took over the mind of Phoenix, to be rescued by Wolverine, Cyclops, and Phoenix herself. This meddling with Phoenix's mind eventually led her to become Dark Phoenix.
Notable members of the Inner Circle include Sebastian Shaw (aka the Black King), Emma Frost (aka the White Queen), Selene, Tessa (both known as the Black Queen at one point), Donald Pierce (aka the White King), Jason Wyngarde (aka Mastermind), Harry Leland, and Trevor Fitzroy. It has appeared in numerous incarnations throughout the X-Men titles and other Marvel series.