The Green Death, renamed The Red Death, is the main antagonist of the novel How To Train Your Dragon and its 2010 animated Dreamworks film adaptation of the same name. It is a giant evil dragon.
The Green Death was a Seadragonus Giganticus Maximus male who during the First Dragon War led the Dragon Army under his then name, Merciless. He was eventually defeated and banished from the Barbaric Archipelago by Hiccup Horrendous Haddock I who threatened to use the power of the Dragon Jewel. Over the next millennium, Merciless became the Green Death, a terror of the sea. He had been at sea so long, that legends told he had forgotten his old life as Merciless.