In How to Train Your Dragon, the dragon attacking Berk at the climax is the Red Death.
The Red Death is a gigantic dragon that controls the nest where the smaller dragons take food gathered from raids. The film reveals that the dragons’ attacks on Berk are driven by fear: they are compelled to bring food to this enormous predator or face being eaten themselves.
Hiccup and the young Vikings confront the Red Death after learning to ride and cooperate with dragons. Their coordinated attack demonstrates the new relationship between humans and dragons, while Toothless and Hiccup take the leading role in defeating the creature.
The Red Death is sometimes confused with the Bewilderbeast, another giant dragon from How to Train Your Dragon 2. They are different characters in different films. The Red Death is the first film’s climactic antagonist, while the Bewilderbeast is connected to the second film’s conflict and dragon control.