Complex sentence: Demona had secretly studied with the Archmage as his apprentice and had developed an aptitude in sorcery, and due to her aforementioned immortality, she has also become quite adept at understanding and using advanced technology.
Split into simpler sentences: Demona had secretly studied with the Archmage as his apprentice and had developed an aptitude in sorcery. Also, due to her greatly extended lifespan and experience, she has also become quite adept at understanding and using advanced technology.