The Green Death is the name of the Seadragonus Giganticus Maximus appearing in the first and mentioned in the ninth book of the How to train your Dragon book series.
The Green Death arrives on long beach with a smaller Seadragonus Giganticus Maximus (which it eats later on). Hiccup is sent to meet with it and asks if it comes in peace or war as he is the only one who can speak Dragonese. When Hiccup says his name the dragon shivers subconsciously it seems however that Hiccup did not notice it nor did the Green Death itself. The dragon says he comes in peace but that he's still going to eat them all regardless. The Green Death and Hiccup discuss Singing Suppers, but during the conversation the Green Death moves in to eat Hiccup. Just as this happens the Green Death yawns, and decides he's too tired to eat Hiccup right now, stating that he should come back in a few hours to be eaten. Hiccup then leaves and tells the tribe that the Green Death, while coming in peace, will still eat them.
With the help of the other Hooligan and Meathead boys and their dragons, Hiccup creates a plan to kill the Green Death with the help of a second Seadragonus Giganticus Maximus, the Purple Death, which they trick into helping them. In the following battle the Purple Death is killed but the Green Death is fatally wounded. In anger, the Green Death swallows Hiccup whole. Fortunately, Hiccup avoids being digested by getting caught on a Roman spear stuck in the dragon's throat. While inside the Green Death, Hiccup discovers the fire holes (a dragon's source of fire) and blocks them with his helmet. Due to this, the Green Death blows up when it tries to burn Stoick, Hiccup and Toothless. Its death makes Hiccup a hero in the eyes of his tribe.
Wearing a fireproof suit made from dragon skin, Hiccup descended into the fire pit below Flashburn's School of Swordfighting. There Hiccup met the Wodensfang who revealed that the Green Death was known as Merciless, and was the leader of the Dragon Army in Hiccup Horrendous Haddock I's time. Merciless foresaw the future and found out that he was to be killed by a human boy named Hiccup. Merciless then sent the Wodensfang to kill Hiccup the First, but was instead betrayed when the Wodensfang discovered the good in Hiccup's heart. The Wodensang stole the Dragonsword and the Dragon Jewel from the very fire pit Hiccup and himself were speaking in and gave the stolen gear to Hiccup the First. Hiccup the First, using the Dragon Jewel ordered Merciless to disband the Dragon Army and become a loner. When Hiccup the Third and the Wodensfang spoke they both saw the irony in the situation as Merciless was eventually killed by a boy named Hiccup.
- The Red Death seems to be inspired by this dragon, seeing how both of them are huge dragons that were killed by Hiccup in an explosion.