Screaming Death's Siblings

The Screaming Death's Siblings are a trio of adolescent Whispering Deaths who are the children of the Screaming Death's Mother and Groundsplitter and the siblings of the Screaming Death.

Biography
The sibling's eggs, along with that of the Screaming Death, were laid by the Screaming Death's Mother on Outcast Island. During the events of Live and Let Fly, Savage, Mildew, and a group of other Outcast soldiers placed these eggs in caverns under the Isle of Berk, where they would soon hatch.

During the events of Tunnel Vision, the tunneling of the Whispering Deaths and Screaming Death cut off Berk's water supply, leading to a water shortage. When Hiccup and Toothless entered the caverns to see where the water went, they were chased out by the three Whispering Deaths, who then proceeded to tunnel up towards Berk. When the three erupted from the ground, they caused utter chaos, with Stoick and Gobber helping the village fight back. Snotlout and Astrid were able to chase off one by having Stormfly lead it into the forest and hitting it in the face with a tree pulled back by Hookfang. The second was defeated by Fishlegs and Meatlug, who tail-bludgeoned it out of the sky. The third was defeated by Stoick and Gobber working together. Combined, the Riders were able to chase the Whispering Deaths off.

However, the Screaming Death soon erupted from the ground and began its attack, calling back its siblings with its powerful scream. Despite this, Hiccup managed to defeat the massive dragon, with its siblings carrying it away. At the end of the episode, the siblings were shown bringing fish to the Screaming Death in order to help it recover.

At some point, the Outcasts likely recaptured the siblings, as they were next seen in pens during the events of Cast Out, Part 2. They were later freed by Alvin and Mildew and used in the battle against Dagur. Once their mother was freed from Dagur and the Screaming Death arrived to meet up with them, they, along with the Screaming Death and their mother, flew away from Outcast Island to find a new home.

Trivia

 * Despite several eggs being seen under Berk in Live and Let Fly, only three of them seem to have hatched, alongside the Screaming Death. It is unknown why this is, though it might have been a production decision to avoid giving the Riders impossible odds.