Complex sentence: Twenty years later, they still wage hopeless resistance against the all-powerful Voldemort, clandestinely helped by Severus Snape-in this reality still alive and still teaching at Hogwarts.
Split into simpler sentences: Twenty years later, they still wage hopeless resistance against the all-powerful Voldemort. They are clandestinely helped by Severus Snape-in this reality still alive and still teaching at Hogwarts.