"A View to a Skrill" description
Here is the description of episode “A View to a Skrill, Part One”:
When the dragon riders discover a frozen Skrill, a lightning-powered dragon revered by the Berserkers, they scramble to get it off Berk. But when the Skrill is thawed and unleashed, Hiccup and Toothless are faced with a battle between both the Skrill and Dagur’s Berserker armada, which could prove to be more than Hiccup and his Night Fury can handle.
From howtotrainyourdragon.com site.