Complex sentence: Harry and the others arrive at the scene just as Dumbledore disapparates with the their past selves and they each use the Patronus Charm to scare off Umbridge's Dementors.
Split into simpler sentences: Harry and the others arrive at the scene just as Dumbledore disapparates with the their past selves. They each use the Patronus Charm to scare off Umbridge's Dementors.