Complex sentence: They are an offshoot of the evolutionary process that created sentient life on Earth, the original instigators of this process the alien Celestials intended the Eternals to be the defenders of Earth which leads to the inevitability of war against their destructive counterparts, the Deviants.
Split into simpler sentences: They are an offshoot of the evolutionary process that created sentient life on Earth. The original instigators of this process the alien Celestials intended the Eternals to be the defenders of Earth which leads to the inevitability of war against their destructive counterparts, the Deviants.