Agatha Harkness has a long and convoluted origin story. She was born into a world where magic was both a gift and a curse. In Salem, she witnessed the horror of the Witch Trials first - hand. This made her very cautious yet also ambitious. As a witch, she was constantly in danger of being discovered and killed. So, she used her wits and magic to not only stay alive but also to learn more about magic. She traveled far and wide, learning from different magical traditions. Her origin story is full of mystery and intrigue as she has interacted with various powerful entities throughout her long life, all of which have contributed to her being the powerful and devious witch we see in the Marvel stories.
One great 'Agatha Harkness Fanfic' is 'Agatha's Ascent'. It really delves into her backstory in a creative way. The author does an excellent job of exploring her character's motives and relationships within the story. It has some really engaging dialogue that keeps you hooked from start to finish.