Dragon Island is the supposed home of all dragons in the film version of How to Train Your Dragon. It is said that the Vikings have been searching for the Dragons' nest since they settled on the island of Berk. After the events of the movie, it was presumed to be uninhabited, but it was soon found to still be inhabited by various dragons in The Night and the Fury.
"Hidden behind a veil of perpetual fog, Dragon Island was once the home of — and now the final resting place to — the massive and fearsome dragon tyrant, the Red Death. For centuries, the seafaring and warmongering Vikings of Berk launched countless campaigns to locate and raze the fabled Dragon Island in the hopes of ending the regular dragon sieges on their homeland… only to return empty-handed time and time again. But where previous generations failed, Hiccup and Toothless succeeded when they followed a strange signal heard only by dragons to the caldera at the heart of the volcanic Dragon Island. There, from deep within a pool of lava, the Red Death commanded all dragons to raid Viking villages and return with all stolen foods to feed its ravenous appetite. Upon learning of Dragon Island’s true location, Stoick the Vast led his Viking army to attack the Red Death, and nearly perished before a timely rescue from Hiccup, Toothless, and their fellow dragon riders. Berk’s victory over the Red Death came at a cost, but that sacrifice was far outweighed by the truth proven by Hiccup and Toothless: humans and dragons can and must work together for the betterment of all."
―Hiccup's Map from Race to the Edge
The Nest is located on an island assumed to be far off from Berk (it is shown to be on the corner of the Vikings' maps). There is a perpetual heavy mist that enshrouds the sea in the surrounding area, preventing anything, save for a dragon, from finding it. The Vikings, under the leadership of Stoick the Vast, were supposedly the first generation of Vikings to be able to find it.
The island itself is simple in geography: it comprises jagged rock structures jutting out of the waters, a beach of gravel, and one, sole volcano. Once one bypasses the mist, the chunks of stone scattered about prove to be more than enough of a challenge for most Vikings to navigate through safely, as well as anything else without thorough knowledge of where to go. The beach itself is peculiar in that, rather than being covered in sand, it is composed primarily of what appears to be pebbles. As a result, making contact with the ground on the island of the Dragons' Nest amplifies sound rather than muffling it as would be the case with normal sand, often tipping off the inhabitants of the island to any intruders. When Stoick the Vast stepped onto the island for the first time, all dragon activity seemed to stop immediately. Lastly, the volcano of the island, which, from the base, appears to be nothing more than a mountain, provides the physical structure of the Dragons' Nest. Along the caverns and tunnels that wind around the periphery and inner layers of the volcano presumably live almost all dragons seen in the film. Within the pool of lava in the center, however, lives a Red Death. Though dragons have been long known to gather food from the Vikings or otherwise, it was only recently discovered that this food is delivered to the mouth of the volcano so that the Red Death may feed. Any dragons that bring back a particularly small catch or nothing at all are eaten by it.
In both the original film and the game, School of Dragons, visitors coming by air or ship must pass through Helheim's Gate, which itself is a very treacherous area to navigate.
In School of Dragons, the volcano has been occupied again by a Green Death. The volcano has active magma, and many caverns and chambers within the mountain. Between the activities of the Green Death and the volcano itself, there are offshore volcanic vents spewing toxic gases out into the atmosphere around the island and out into the Archipelago. Dragons are able to tolerate the noxious fumes, but humans cannot.
Also visible in the School of Dragon's iteration of the Island is the skeleton of the deceased Red Death from the original film, as well as the broken up debris of Berk's Ships.
For three hundred years, dragons lived on the island and made raids on Viking islands such as Berk. On the island in the mountain, the Red Death demanded all the dragons to give it all their food or he would eat them. Hairy Hooligan chief Stoick the Vast and his vikings attempted to find the island before the ice sets, to no avail. The Island was then discovered by Stoick's son Hiccup, and Astrid Hofferson when Hiccup helped and trained the Night FuryToothless. Stoick then led the tribe using Toothless to find their way through the fog bank. Once there, they attacked the dragons, scaring them away until the Red Death burst out of the mountain, destroying the ships. However, the Red Death was then defeated by Hiccup and Toothless. Most of the dragons left the island and settled on Berk.
When Alvin and his Outcasts returned to Berk, Alvin took Hiccup to the Island for him to prove that he was the "Dragon Conqueror". It was there that Alvin realized that the Hooligans rode dragons. Alvin and his men were then marooned after being defeated by Stoick, Hiccup and their Dragon riders. It is unclear as to how Alvin and his outcasts returned to their Island.
A stowaway named Hroar arrives at Berk and takes a great interest in Berk's dragons. However, the dragons seem to be on bad behavior, and only Hiccup suspects something suspicious with the new arrival. Hroar asks Astrid to show him Dragon Island, and when they arrive, his real intentions come into play.
The game Rise of Berk has the location called "Dragon Island" that is searchable by Toothless. However, based on the dragon eggs found at this location it the game, it is possible that the game is referring to Valka's Mountain (for example, users can find Lump, Thump, and Cloudjumper here).
In June 2016, School of Dragons released the Expansion "Return to Dragon Island", which centered around Dragon Island. In the game, this expansion takes place after the death of Stoick. In the expansion, the player must figure out why and from where there is a toxic mist spreading across the Archipelago. Eventually the player finds out that it is from volcanic fumes emitted by the volcano on Dragon Island. Throw many quests, the player interacts with the NPC character Harald Forkbeard, the Green Death currently residing in the volcano, other NPCs, and an Eruptodon which the player eventually trains at the end of the expansion.
Also in the expansion, the player trains an adult Singetail.
A Gronckle was eaten by the Red Death when it didn't give enough fish for it in the original How to Train Your Dragon (film). The Gronckle who was eaten by the Red Death, looked smaller than other Gronckles. It might be that this Gronckle was younger than others of his species.
Two Gronckles made a brief appearance, fighting each other, until Hiccup comes into the scene by feeding them rocks. - Dagur also managed to kill one in The Night and the Fury.
This species of dragon was formerly enslaved by the Red Death, but after its defeat, they and the other dragons were freed in the original How to Train Your Dragon (film).
A wild Deadly Nadder was found when it confronted Astrid, who stayed in its blind spot. The Nadder then throws its spines, which Astrid successfully avoids, and flees. A Nadder's spine was found by Dagur the Deranged who admitted that getting its spine in his leg was 'awesome' in The Night and the Fury.
A Monstrous Nightmare was encountered by Ruffnut. The Nightmare chased her and blasted fire at her. Somehow, she escaped it.
Dagur also managed to kill another Monstrous Nightmare, as he showed Hiccup its skull mounted on a stick. Another Nightmare was shown when Hiccup is 'dragon hunting' with Dagur. Dagur thought it was a Night Fury, yet Hiccup told him that it was too big. When it was revealed to be a Nightmare, Dagur tried to shoot it down anyway until Hiccup stopped him, saying that the Night Fury would find them if they do. However, this was probably just an effort to save the Nightmare from being killed in The Night and the Fury.
The riders take the baby Thunderdrums to Dragon island, where they got attacked by bigger dragons, so Thornado stays with them to help them survive in Bing! Bam! Boom!.
It is interesting to note that, although the Red Death is initially depicted to live bathed in lava, when it later breaks its way out of the mountainside to face the Vikings, the lava is nowhere to be seen. However, it is possible it climbed out of the lava, into the tunnel.
It is unknown why the dragons returned to feed the Red Death instead of just flying away. The dragons didn't apparently need anything from the Red Death, so they could simply have left it to starve, seeing as it was lodged in the volcano. One possibility is that the Red Death had some undiscovered form of dragon telepathy that the dragons could only ignore when their lives were in imminent danger, and was only able to be countered by having a human rider.