- "It's a Seadragonus Giganticus Maximus, and a particularly big one, I'd say. Very cruel, very intelligent, ravenous appetite."
- ―Old Wrinkly in Book 1
This dragon is the largest (perhaps also the rarest) of all known dragon species; so large, in fact, that it has frequently been described as "blotting out the moon" when in full flight. Seadragonus Giganticus Maximus are blue, green, or purple, and usually covered in marine corals, anemones, and barnacles that act as natural armor.
- "Vast ... Gigantic. Stupendously enormous. Five times as big as the Big Blue Whale."
- ―Old Wrinkly in Book 1
However as it is discovered in How to Betray a Dragon's Hero, the size of these dragons change in line with their age. They start off extremely small, grow to an impossibly huge size, and sometimes shrink down to the earlier extremely small size (it is unknown why the similarly aged Green Death and Wodensfang were so differently sized -.possibly dietary differences control it).
- "Seadragonus Giganticus Maximus emerge from the egg as small as nanodragons. Over thousands of years they grow larger and larger until they are many times bigger than the big blue whale, as vast as mountains, as huge as islands."
- ―The Incomplete Book of Dragons
Their eyes are described as "Crocodile eyes". In fact, fully grown adult Sea-Dragons have eyes in the back of their heads.
- "... that adult Sea-Dragons do indeed have eyes in the back of their heads that begin life as spots or moles, and grow into eyes as the dragon grows, and then as the dragon ages and gets smaller, the skin closes over them, and buries them, and they dwindle into nothing again."
- ―How to Fight a Dragon's Fury
The blood of the Seadragonus Giganticus Maximus is green.
Seadragonus Giganticus Maximus are known to possess powers of telepathy and limited ability to see the future. They can breathe fire continuously for at least an entire night, which is powerful enough to dramatically alter landscapes. One Sea-Dragon, Wodensfang, has demonstrated possession of retractable teeth.
Informally referred to as the 'king of dragons', these dragons are nigh-invincible and virtually invulnerable, possessing an immense amount of raw strength and power. As per How to Fight a Dragon's Fury, a full grown Seadragonus Giganticus Maximus have many extraordinary physical features and powers including, but not limited to, eyes at the back of the head, an unparalleled arsenal of fire-breathing abilities including fire blasts that explode on impact (vaporizing massive objects) and eyes that literally catch fire and flame. It is also suggested that dragons of this type may also have abilities unique to the individual specimen; not always replicated in their kin (for instance, the dragon Furious has demonstrated the ability to emit lightening bolts from his eyes). These dragons have also demonstrated strong (albeit limited) precognitive abilities; seeing through the barrier of time into the past and the future to perceive events that have not yet come to pass. They can telepathically communicate with other sea dragons, and their eyes have hypnotic qualities. Most sea dragons can also speak fluently in Norse, the Viking language - "when they so choose".
The armament of the Seadragonus Giganticus Maximus is quite extensive.
- "The Sea Dragon is the most well-defended creature that has ever lived on this planet. Its skin is over three feet thick in places, and is so encrusted with shells and barnacles that it almost has the effect of armour.
It is also the most well-armed creature that has ever lived on this planet and its razor sharp claws and teeth can rip open its own iron crust as if it were made out of paper. ..."
- ―Book 1
The promise of a Seadragonus Giganticus Maximus is considered to be unbreakable, although in practice there is nothing to ensure it.
The Green Death/Merciless
- Main article: Green Death (Books)
The Green Death (or formerly known as Merciless) is the most well-known Seadragonus Giganticus Maximus. It claims, in the book, that it is an artist by eating other dragons, and is very proud of its size. In his youth, Merciless was the leader of the Dragon Rebellion, until Hiccup Horrendous Haddock I, using the Dragon Jewel, made him leave the Archipelago and never return, or else his fate--to be killed by a boy named Hiccup--would come to pass. A thousand years later, he returns to the Archipelago and is killed by the cunning of Hiccup the Third.
The Purple Death/Horror
- Main article: Purple Death (Books)
The Purple Death (also called The Purple Horror) is one of three Seadragonus Giganticus Maximus to appear in the first book of the How to Train Your Dragon book series. It is said to be larger than the Green Death, but is eventually killed by it. Thuggory the Meathead is the first to notice the Purple Death after a terrible storm washes it up on the Isle of Berk. The Hooligans try to destroy both it and the Green Death by making them attack each other, which, despite the Purple Death being supposedly larger, resulted in the Green Death killing it.
- Main article: Furious
Furious was Hiccup the Second's dragon, sometimes referred to as his "brother". After Hiccup was killed, he was captured and used by the Berserks as an executioner, and was turned bitter and evil over the years. He currently leads the Dragon Rebellion, whose goal is to exterminate the human race (Hiccup Horrendous Haddock III in particular).
- Main article: Wodensfang
It is revealed in How to Betray a Dragon's Hero that the Wodensfang is a Seadragonus Giganticus Maximus. He was the dragon of Hiccup Horrendous Haddock I. He stole the Dragon Jewel and gave it to Hiccup I, stopping the first dragon rebellion. When Hiccup III came around, he gave Hiccup III the Crown of the Wilderwest in the Fire Pit. Later, he makes a deal with the Dragon Furious that he will betray whoever becomes King of the Wilderwest to the Rebellion.
- Main article: Toothless (Books)
In How to Betray a Dragon's Hero, it is revealed that the Toothless from the books is also a Seadragonus Giganticus Maximus by the Wodensfang. He is Hiccup Horrendous Haddock III's dragon, a tiny, stuttering, toothless Common-or-Garden--so everyone thought. In time, Toothless will grow to the size of a mountain, possibly gain teeth, and then shrink again in his old age. He is the "Best Thing" of the King's Lost Things, according to himself.
Unnamed Seadragonus Giganticus Maximus
- Main article: Unnamed Seadragonus Giganticus Maximus
Another Dragon appears in How to Train Your Dragon on the shores of Berk, but is quickly eaten by the Green Death.
- All three Hiccups have had a Seadragonus Giganticus Maximus ally. Hiccup Horrendous Haddock I's was the Wodensfang, Hiccup Horrendous Haddock II's was Furious, and Hiccup Horrendous Haddock lll's was Toothless. All of them have stitches over their hearts.
- The Red Death, the film counterpart of the Green Death, was originally called a Seadragonus Giganticus Maximus on the School of Dragons website. However, the Red Death has now been deemed its own species.