Complex sentence: Harry, who is mentally connected with Voldemort, manages to see this in a dream and is able to warn the Hogwarts authorities and Mr. Weasley is saved just in time and sent to St Mungo's Hospital for Magical Maladies and Injuries.
Split into simpler sentences: Harry, who is mentally connected with Voldemort, manages to see this in a vision and is able to warn the Hogwarts authorities. Mr Weasley is subsequently saved just in time and sent to St Mungo's Hospital for Magical Maladies and Injuries.