Complex sentence: The few caretaker dolphins who had been left behind on Earth were not entirely defenseless; they were protected by their creation called the Guardian--a gigantic, sentient being made of a crystalline substance which projected a forcefield over the entire planet.
Split into simpler sentences: The few caretaker dolphins who had been left behind on Earth were not entirely defenseless; they were protected by a creation of theirs they called the Guardian. It was a gigantic, sentient being made of a crystalline substance; it projected a force field over the entire planet.