Agatha Harkness was in reality a member of an ancient race of creatures known as the Tan-Ed-Drul. It is this race that inspired the Greek tales of the seven-headed Hydra, and it is also this race that was responsible for the cataclysm that destroyed Atlantis.
In the early 21st century, one of these Hydras assumed human form and took the name Agatha Harkness. As Harkness, she infiltrated S.H.I.E.L.D. as a psychiatrist and used her new-found connections to gain access to the government think tank known as the Baxter Building. She began conducting sessions with members of the Fantastic Four with the alluded intention of "maximizing the team's efficiency".
During this time, Agatha Harkness replicated her own form, creating a gestalt team of superhuman beings known as the Seven. The Seven began operating in New York City, performing various deeds with the intent of one-upping the Fantastic Four. This brought them into several engagements with the team, who were already suffering doubts among themselves due to Harkness's manipulation.
It was Susan Storm who eventually learned the truth about Harkness, but when she tried to expose her, the Seven had Susan beaten and tossed into the New York Harbor. If Namor had not been around to save an unconscious Sue, she would certainly have drowned. Harkness used her feminine wiles to seduce Johnny Storm. Catching him at a moment of vulnerability, Agatha injected him with a sedative and was able to kidnap him.
In her replication form as the Seven, Johnny was brought back to a hidden Tan-Ed-Drul hatchery concealed beneath Nursery Two at a Baxter Building installation in Pinhead Buttes, Oregon. Harkness planned to use Johnny's powers to fire the replication process of the Tan-Ed-Drul. Susan and Namor regrouped with the Fantastic Four and traveled to Pinhead Buttes where they engaged in a final battle with the Seven - a battle that cost the villains, and presumably Agatha's persona, their lives.