Complex sentence: They realize that the demons are going to set up their hell on earth and take back what is theirs and rule the universe once again, as creatures climb out of the gate to capture two of the kids in order to sacrifice them.
Split into simpler sentences: They realize that the demons are going to set up their hell on earth and take back what is theirs, in order to rule the universe once again. Creatures climb out of the gate to capture two of the kids in order to sacrifice them.