Warrior Demoness by Fran Heckrotte

Lesbian Fiction: Paranormal - The Illusionist Series Book 6 - She was Sabnock, a demon, who like the Phoenix, lived and died many times because she chose to live amongst mortals rather than spend eternity babysitting the legions of the Underlord. There were no longer battles to be fought in the Underworld so the ex-commander left her realm to live with the humans as human. Falling in love, she now had to choose between her vow to live and die as a mortal or love and live as a demon, not knowing if her lover could accept the truth. The wrong decision would condemn her to a life of loneliness-and for a demon life was eternity.