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Quentin is first introduced via rumors of a killer who sends flowers to women, who he targets soon after - abducting and killing them, though at this point no one knows his true name or identity.
It is not until later that Hawke discovers his/her own mother, Leandra Amell, is being courted by a man sending flowers, raising suspicion - tragically by the time Hawke and his crew track her down it is too late.
Quentin, a Blood Mage who was emotionally unstable after losing a lover, had taken Hawke's mother due to her face resembling that of his beloved - sitting her on a chair facing away from Hawke and the crew it seems at first she can be saved..
Yet Quentin soon reveals the horror when he helps Leandra up to show that he has actually mutilated her and transformed her into a "perfect" woman via Blood Magic - effectively turning her into a zombie, stitched together with the body parts of his other victims.
Quentin proceeds to summon a legion of demons (who presumably had a hand in his madness) as Hawke and his/her crew fight and ultimately slay the mad mage.
Tragically without Quentin's magic, Leandra dies, but not before she manages to tell Hawke how proud she was of her son/daughter before passing on.
Later it's revelated that Orsino supported him by giving him access to books about forbidden magic and hiding his existence in secret.