Sabine was imprisoned for 83 years for being in a secret love relationship with Hamish, a Dark Fae — an act that defied Fae law and was considered treasonous by both clans. Hamish's clan was arranging a marriage for him and the two made plans to elope before they lost each other forever. On the night they were going to make their escape she was met by Light Fae guards who knew about their secret plan and accused her of treason. During her fight with the guards one of them was seriously injured. Sabine was arrested and convicted of treason, and sent to prison for the rest of her life. She did not hear from or see Hamish again. When the Stag Hunt was announced, she volunteered to be The Stag.
Hamish was not present when Sabine was arrested and she was left wondering if it had been her own people or the Dark Fae who had betrayed them. Hamish revealed that his family had found out about him and Sabine, they became furious that he would jeopardize the family name and tipped off the Light Fae about her.
Her Fae species was not revealed, but when she would rapidly whirl her body, she shot sharp poisonous quills from it that knocked out her target for hours (or could as well kill it).
Hamish: Her romantic partner.
Her being able to projectile sharp quills out of her body suggests that she was a Porcupine-like species.