Dragon Island

The Dragons' Nest/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 Nest since they settled on the island of Berk. After the events of the movie, it was presumed to be uninhabited, and yet it was found to be inhabited by various dragons.

Description
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.

Geography
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.

How to Train Your Dragon
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 Fury Toothless. 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 bursted 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.

In Dragons We Trust
When a series of incidents were blamed on the dragons, Stoick ordered Hiccup and his fellow riders to take their dragon to the Island to prevent more incidents.

Alvin & the Outcasts
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.

Dragons: Defenders of Berk
Following the Outcasts attacks with wild Dragons, Astrid arranged a training mission for the dragon riders.

Inhabitants

 * Red Death (formerly): The Island is formerly home to the Red Death, a Seadragonus Giganticus Maximus that enslaves other dragons. But the beast was defeated by Hiccup Horrendous Haddock III and Toothless.
 * Gronckle: A Gronckle was eaten by the Red Death when it didn't give enough fish for the Seadragonus Giganticus Maximus. In the Defenders of Berk episode "The Night and The Fury", 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 movie, 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. Several of them also attacked Bing, Bam and Boom. They were also recruited by Fishlegs and Astrid to defeat the Screaming Death.
 * Hideous Zippleback: Some Hideous Zipplebacks served the Red Death, one notably being accidentally eaten by the dragon when chasing Toothless, Hiccup and Astrid. Tuffnut, accidentally thought a wild Zippleback to be Barf and Belch, which chased him until they got stuck between a tree. Many wild Zipplebacks attacked the triplet Thunderdrums.
 * Deadly Nadder: This species of dragon was formerly enslaved by the Red Death, but after its defeat, they and the other dragons were freed. 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'. A blue-and-red Deadly Nadder attacked the Thunderdrum tripletts. There were also several Nadders used to fight off the Screaming Death.
 * Monstrous Nightmare: Monstrous Nightmares were among the dragons that served the Red Death. 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. A green Nightmare attacked Bing, Bam and Boom.
 * Terrible Terror: The Terrors served the Red Death. A small group of Terrors attacked Snotlout, causing him to fall into a small lake. They also attacked Dagur when he was about to kill Toothless. The_Night_and_The_Fury_Typhoomerang.png
 * Typhoomerang: In spite of being a new dragon, the Typhoomerang makes when it was awaked by Fishlegs when he walked near it.
 * Thunderdrum: 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.

Trivia

 * 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.
 * The Dragons' Nest does not itself exist in the How to Train Your Dragon book series, though it has a counterpart in Wild Dragon Cliff. However, no Seadragonus Giganticus Maximus is known to live there.
 * 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 counterable by having a human rider.