The Sea Hag is the sorceress of immortality, born into the true first human colony of Xanth, 2200 years before the First Wave. When she dies, she can take over any body she wishes, male or female (a fact she discovered after her first death).
When she was known as the Sea Girl, she married a man named Green Horn. He made her life miserable. When he died, she vowed she would make every man's life as miserable as Green Horn made hers.
She is introduced in Golem in the Gears, serving as the main antagonist as she prepares for her latest body switch with Rapunzel. Her efforts are halted through the efforts of Grundy Golem, resulting in her banishment to the Brain Coral's pool.
She escapes 33 years later during the events of The Dastard, when a glitch ejects her, as well as several other people. Despite her best efforts and an alliance with the Dastard himself, her soul is finally banished from Xanth for good by the actions of Melody, Harmony, Rhythm, and Sim from Ptero after switching with their Xanthian younger selves.
Her origin as a pre-First Waver is revealed in Question Quest, and her full history (including the discovery of her talent) is revealed in The Dastard, while possessing one of her latest victims.