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 the Green Death's past. About a thousand years ago, 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 Wodensfang 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.
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.
- "The great storm lifted both Dragons gently from the sea bed like a couple of sleeping babies, and washed them on the swell of one indescribably enormous wave onto the Long Beach, outside Hiccup's village."
- ―Book 1
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.
In the epilogue of Book 1, an Older Hiccup recalls the Green Death singing the Supper's Song after death.
The Green Death is an extremely large, green Seadragonus Giganticus Maximus. He is compared to the Unnamed dragon of the same species who also washed ashore on Berk.
- "The first dragon was averagely enormous, about the size of a large-ish cliff. The second Dragon [Green Death] was gobsmackingly vast."
- ―Book 1
- "The first Dragon was enough to give you nightmares. The second Dragon was enough to give your nightmares nightmares."
- ―Book 1
Stoick further describes the Green Death's size during a Council of War:
- "This Dragon is HUGE. Enormous. Gobsmackingly vast. I've never seen anything like it. This is more of a mountain than a Dragon."
- ―Stoick the Vast
The book goes on further to describe the Green Death's skin, size, and color. He is covered with barnacles which form a sort of natural armor.
- "Imagine an animal about twenty times as large as a Tyrannosaurus Rex. More like a mountain than a living creature - a great, glistening, evil mountain. He was so encrusted with barnacles he looked like he was wearing a kind of jeweled armor but, where the little crustaceans and the coral couldn't get a grip, in the joints and crannies of him, you could see his true color. A glorious, dark green, it was the color of the ocean itself."
- ―Book 1
The Green Death is so large that he also has "twenty-foot back teeth" and his yellow eyes are "as big as six tall men". His breath is also extremely pungent:
- "It was the stench of DEATH itself - a deep, head-spinning stench of decaying matter; of rotting haddock heads and sweating whale; of long-dead shark and despairing souls."
- ―Book 1
The Green Death's claws are both massive and also retractable like a cat's.
- "As the Dragon spoke, he extended his claws, the talons slowly emerging from the thick stumps of his fingers and rising up until they resembled nothing more than gigantic razors, six feet wide and twenty feet long, with points on the end like a surgeon's scalpel."
- ―Book 1
The Green Death is noted to have a roar so loud, it can kill small creatures instantly from fear.
His fire is "bright green" and he seems to have excellent control and precision about where the fire lands and in what pattern.
The Green Death's talons are extremely large yet extremely precise; despite their size, the Green Death is able to separate the spine from a sheep or human vastly small than himself, or even rip the clothes from a person without harming the human.
- 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.
- The game Dragons: Rise of Berk did include a dragon named the Green Death. It was mentioned as being the less agressive cousin of the Red Death. Wether this meant the two individual dragons were related or wether the Green Death was a subspecies of the Red Death was unclear. The game School of Dragons also included the Green Death in its Return to Dragon Island expansion pack, though this time the Green Death was clearly referred to as a subspecies/cousin of the Red Death.