Complex sentence: The dragon riders and Eret ride baby dragons, which are immune to the Alpha's control, and arrive at Berk after Drago's Bewilderbeast has already attacked the village and taken control of the dragons there.
Split into simpler sentences: The dragon riders ride baby dragons back to Berk, as they are immune to the Alpha's control. They find that Drago's Bewilderbeast has already attacked the village and taken control of the dragons there.