Thaegan was a powerful sorceress like her mother. Her mother was a free-spirited sorceress named Tamm who used her magic for benevolent reasons such as healing. But Thaegan was much different as she was spiteful to other living beings, with an interest in the dark arts. As a result, Theagan ran away from home after sheer disgust towards her mother, whom she killed after mastering the dark arts.
Her favorite food is a live raven, ironically assuming a similar form; thus she apparently ate Kree's parents and causing him to flee to the Forests of Silence.
Though the only way she could die is if a single drop of her tainted blood hits the ground, Thaegan can use her magic to create a barrier around herself while not needing to cast a spell. Over time her power grew and, out of hatred for vitality of the life and beauty, turned the city of D'Or into the site of the Lake of Tears with in occupants turned into various creatures. When the Ralads had known of her treachery, they spoke out against her, so she used her magic to render them mute. By the time the Shadow Lord took over Deltora, Thaegan became one of his most powerful allies and her power grew with his dominance.
After hearing of the death of her children Jin and Jod, Theagan tracks Lief, Jasmine and Barda to the Lake of Tears as they nearly obtained the ruby. In fight that followed, Thaegan almost destroyed them were it not for Kree pecking her finger just as she lowered her barrier to blast the heroes, undoing her enchantments with her death.
Deltora QuestIn the anime Deltora Quest, the Shadow Lord revive Thaegan through her ring after a rat brought the item to him. Fully restored after the events at the Shifting Sands, Theagan finds her remaining children so they get revenge, recruiting Hot, Tot, Fie, and Fly to bring Lief's group to her.
However, after being aged by the lapis lazuli and accidentally killing her four children, Thaegan is killed off by Lief cutting her ring finger off with her ring taken by the group to ensure she is not revived again. However, Theagan is revived again by the Shadow Lord, who motivates the sorceress with fear of her beauty being ravaged if she fails him again.
Using the Mirror of Fear she received, capturing them after their hardship in the Maze of the Beast, Theagan attempts to torture Lief's group before killing them once and for all. However, the plan failed as the Shadow Lord's curse takes effect with Thaegan losing her magic while the spell that kept her youthful is broken as she reduced to an old corpse hag. She refused to live on in this state and decided to kill herself with what little magic she had remaining, unexpectedly taking most of her remaining offspring with her as she spent her final moments expressing motherly love to them and the spirits of her deceased children.