The Red Death is a gigantic dragon species in the first How to Train Your Dragon movie. The gender of this individual is probably female (for the one we know of), as it is known as the queen of the Mountain Nest of the dragons.
It does not actively hunt but it has its food delivered to it by other dragons in the nest. It will eat any dragon that does not bring sufficient food; when a Gronckle brought only a single fish, the Red Death from the movie devoured it moments later.
True to the reputation as the 'queen of the dragons', the only grown specimen seen in the franchise demonstrated savage, menacing, and controlling nature that even a deadly Night Fury was frightened of (not unclear whether the other individuals share this personality). In addition, according to Valka, every nests have their queens or kings, indicating that there are numerous Red Deaths in the world.
The Red Death, the second biggest species after the Bewilderbeast, is a colossal dragon; literally its entire body is a weapon itself. The dragon primarily uses its enlarged nostrils (that slightly resemble those of the Deadly Nadders) and hearing in order to hunt down its prey. Its entire body is heavily armored with horns and a coral-like frill, enabling the titanic beast to perforate, piece, and demolish enormous rock cliffs of volcano and smaller mountains entirely. It also has a tail specifically designed to smash into objects.
Not only with brutal destructive power, the queen of dragons has been shown to have an incredible speed and stamina in flight despite its tremendous size, being able to keep up with Toothless for long amounts of time. The dragon also does not have a blind spot, due to having six sensitive eyes, and heavily relies on scent and hearing.
The Red Death's flame jets are enormous and are powerful enough to take down an entire Viking fleet of ships in just one breath. Like other dragons however, the Red Death can't hunt efficiently if its wings are damaged and it's just as susceptible to fire on its insides.
See Fire Types.
Resembling of the Bewilderbeasts, the Red Death is also capable of enforcing smaller dragons to obey even against their wills. Controlling measure is somewhat similar to methods used by the Bewilderbeasts, Valka, and Drago Bludvist: sounds. Like bees, the queen howls sub-vocal homing signals that reach miles away to call up minions. Unlike the Bewilderbeasts and kinder individuals, this Red Death was also harnessing fear positively to enslave other dragons; single stares from six eyes are very enough to diminish smaller dragons' self possessions completely.
Commanding thousands of minions, the Red Death has acute intelligence and keen insight, to notice small events or intruders into the nest. The system to enslave and dominate smaller dragons in the areas at the first stages is unknown.
The dragon, while in magma, seemed commanding the minions to throw and drop gathered foods into exact spot: its mouth, otherwise the foods will be cremated before passing through fangs of the queen. It is also unclear whether it has traits of Tidal Class to be able to manage respiration under thick liquids.
The Red Death showed for his massive size to have amazing strength to lift up Viking ships and can come through a mountain out of dragon island.
The Red Death possesses a unique ability to have a massive intake in air and "suck in" its victims inside its mouth. 
Dragons of the Red Death
The Dragons of the Red Death are a group of dragons that are enslaved and forced to serve a injurious parasitism Red Death. The dragons of the Red Death could be simply called 'domestic dragons', these include the Deadly Nadder, the Monstrous Nightmare, the Gronckle, the Hideous Zippleback, and even the Night Fury. All or most dragons in the movie appear to serve the Red Death. The Red Death has been compared to a queen bee or a brood parasite. It is the leader and all others serve it apparently out of fear of being eaten, although it is unknown for certain how it became the leader of the other dragons. (It is possible that it invaded their home and proclaimed itself as such or perhaps some kind of mind control similar to that of the Bewilderbeast).
In How to Train Your Dragon
Hiccup and Astrid discovered this dragon when Toothless followed a group of dragons to the volcano where the Dragons' Nest was contained. There they witnessed a Red Death eat a dragon that did not bring enough food, deducing later on that this dragon forces all the others to steal food from Berk.
Stoick the Vast got this information out of his son, and soon led a party to where Hiccup had explained, using a restrained Toothless as their guide.
Hiccup helps his friends tame their own dragons to rescue Toothless from the Hairy Hooligans who captured the dragon in hopes of being lead to the Red Death. Hiccup allies his friends with these dragons: Astrid and her Deadly Nadder, Snotlout and his Monstrous Nightmare, Fishlegs and his Gronckle, and Ruffnut and Tuffnut and their Hideous Zippleback. Hiccup then shares a ride with Astrid, and they part for the Red Death's lair where they'll find Toothless and the Hairy Hooligans.
The Hooligans had opened fire on the side of the volcano and broke a hole in it, freeing the dragons inside and waking the Red Death. The dragons had gone, flying away, in fear of the wrath of the Red Death. The Hooligans, though, were not aware of this until it existed the mountain, plowing through the wall. The Hooligans fired at the dragon with their catapults, which only prompted it to smash them. Terrified, they attempted to retreat to their ships, but it destroyed them all before most of them could even reach them. Before it could kill them, however, Hiccup and his allies arrived on their dragons and diverted its attention.
Hiccup and Stoick later help free Toothless from his bonds, and together, Hiccup and Toothless bombarded the Red Death with their fire. The duo lead the Red Death to the sky, into the clouds. The Red Death had lost track of them in the clouds, as they were using them as cover. Toothless shot the Red Death in a sneak attack, and then disappeared into the clouds; he repeated this multiple times, until making the Red Death being enraged, and set the surrounding area in fire in circular motions. Toothless' synthetic tail caught fire, and they had to hurry their plan.
Hiccup drove Toothless downward, and the Red Death followed close behind. Toothless flew as fast as he could manage, though the Red Death always strayed closer. Just before the Red Death was about shoot fire at them, Hiccup turned his dragon and Toothless shot a bolt of thunder into the Red Death's mouth, overheating it. With the straying embers off Toothless' tail, the wings of the Red Death caught fire, and burned holes into the skin. The Red Death lost control and crashed into the shoreline, exploding to its death on impact.
With the Red Death now gone, the dragons are now free and no longer have to raid the island of Berk of their food anymore, and the Hooligans were taught to live with them in peace.
In Book of Dragons
The Red Death was probably the large 6 eyed grey dragon in the Book of Dragons.
Film Developer Description
"This ultimate monster, seen pursuing Toothless and Hiccup, is the most vicious. According to legend, she lives 2,000 years and, before she dies, lays 3,000 eggs. The hatchlings fight each other to the death, a process that can take up to 100 years, until only one survives. “All dragons have a blind spot,” Sanders says. “Except this one. That is one reason we gave her six eyes instead of two. The other reason is that we wanted her to be just generally upsetting. This one is our Godzilla.”"
- While clearly meant to serve a similar role as the Green Death from the book, the antagonist, the Red Death has quite a few differences from the Green Death: it doesn't appear to be a sea-dragon and instead seems to be more dinosaur-like, it lives in lava, and whereas the Green Death is almost a victim of fate, the Red Death is largely responsible for all the trouble caused in the movie.
- In the Art of How to Train Your Dragon book from the film, the original designs and sketches all depicted it as a far more sea-dragon-inspired creature to the point of it being eel-like and covered in coral. The final design, however, went with a combination of in-book traits and kept the aforementioned coral-like spikes.
- The concept image of the Red Death was more slender and oceanic, faithfully reflecting images of the Seadragonus Giganticus Maximus in the books; literally being concept arts of the movie edition of the Seadragonus Giganticus Maximus. Some of earlier concepts had multiple crab-leg-like smaller limbs, or feeler-like tentacles resembling those of sea anemones on tip of chin, coral-like projections as armours on entire body, and a number of luminous organs like those of jellyfish and deep sea organisms, making it unlike any sea-dragons previously seen in the books. Because of these aspects, the concept dragons' heads to necks somewhat resemble of dragons in eastern lore.
- This dragon is the largest species seen in the film by a wide margin. It has a large club like tail and thick skull. An odd trait of the this dragon is it has six eyes, three on each side, and can fly despite being so big and having small, beat-up wings.
- In the movie, it's shown to have a more dinosaur-like head and mouth with a rhinocerous-like horn in its nose and a frill, while in the book its head and mouth resemble more of a classic European dragon.
- According to developer commentary, the final fight with the Red Death was modified in its last scenes. After Hiccup and Toothless send a bolt of fire into its stomach and it begins to explode, they fly down its spine and away to safety. In this scene the dragon's size was tripled to make the scene more dramatic.
- In the movie, Tuffnut makes an allusion to Beowulf by calling the Red Death 'Bride of Grendel'.
- The Red Death is one of the few dragons that was not known to Berk, with no references to it in the Dragon Manual and shown through the sheer surprise of Gobber and Stoick when it bursts through the rocks.
- The Red Death is one of the few dragons that is not categorized in any of the Dragon Classes during the Book of Dragons special. It was classified as a Stoker Class in the book Guide to the Dragons created for the second movie.
- The Red Death is the only antagonist to actually be killed in the films.
- Despite being called the Red Death, the only red parts on its body are the small spikes on its sides and the ridges of its wings.
- This is the only species that had been confirmed so far to have multiple pairs of eyes on a single head.
- ↑ 1.0 1.1 Red Death on Dragonpedia. Retrieved on 20 June 2014
- ↑ 'How to Train Your Dragon' takes flight - USA Today - Page 7 of 7