Inferno

Inferno, also known as the Dragon Blade, is a retractable sword that serves as Hiccup Horrendous Haddock III's main weapon. The sword has several features, most notably the ability to become inflamed.

History
As Hiccup began exploring further away from Berk, he began to discover more species of dragons and new enemies that he needed to defend against which inspired the creation of the Dragon Blade, also known as Inferno. It was originally created for the purpose of removing hardened Death Song amber from the Riders and their dragons. Later in its use Hiccup discovered that it could be used to pacify wild dragons, who tend to find it strangely hypnotic.

Dragons: Race to the Edge, Season 3
In "Tone Death", to free Tuffnut's Chicken of Garffiljorg's Death Song amber cocoon, Hiccup, reminded of Monstrous Nightmare gel being able to free the entrapped from the amber, takes a knife, applies gel from Hookfang and has Belch ignite the gel, giving him a flaming knife that was able to free Chicken without harming her. This gives Hiccup the idea of using a larger blade with a hollowed hilt to hold the gel and manual igniter to create a weapon capable of freeing the Riders and their dragons should that become entrapped in the amber which did indeed happen. Astrid commented that if the blade was twice as big it would be a really cool sword. Hiccup dubbed his creation "Inferno," but the other Riders were unsure of the name. Hiccup said he would come up with a different one, as well as lengthening the shaft, adding another canister for Nightmare gel, and making the gas canister airtight. Getting excited, Hiccup takes off, leaving Astrid saying that's the last they'll see of him that week. It was later shown that Toothless had to blow out the flame to deactivate the fire, as seen in "Family on the Edge" when Dagur shows up at Hiccup's hut and is seen inspecting the inflamed blade.

Dragons: Race to the Edge, Season 4
In "Midnight Scrum", Hiccup used Inferno to cause a sheep stampede to scare off Ryker Grimborn and a pair of Dragon Hunters. He tried to use it against Krogan but was disarmed quickly. He also tried to calm down the Small Shadows with it but failed. Hiccup then used Inferno to defend himself, Fishlegs, and Mala from the Fire Terrors of Caldera Cay's volcano in "Out of the Frying Pan", only to discover that the dragons eat fire. He also was seen trying to use it to possibly pacify the Triple Stryke, Sleuther, in the episode "Blindsided", but its flame was extinguished by the rain.

Dragons: Race to the Edge, Season 5
Hiccup had improved upon Inferno, as he said he would, and extended the blade. Its hilt looks more like it does in How to Train Your Dragon 2. He now used it more for combat as he fought Berserkers in "Something Rotten on Berserker Island" and was seen training with Astrid in "Dawn of Destruction". He also used to free Stormfly from Death Song amber in "No Dragon Left Behind". He then used it as a torch in an underground cave and to break a wall of amber in which Garff trapped himself.

Dragons: Race to the Edge, Season 6
Though the Inferno stays more or less the same in Season 6 as in previous seasons, Viggo Grimborn makes his own fire sword with some modifications in "Triple Cross". These include retractability and use of Zippleback gas. It mirrors the Inferno's design in the second movie, implying Hiccup was likewise inspired by Viggo's sword and modified the Inferno.

How to Train Your Dragon 2
Hiccup first used the Dragon Blade when Stormfly had been captured in a net, fired by Eret, Son of Eret and his crew, to intimidate the Dragon Trappers and cut Stormfly free from the ropes of the net.

He later used it in front of the trappers again to draw their attention to the fact that he was still armed by igniting the blade and, after surrendering it to the trappers, tricked one of them into releasing the Zippleback gas and igniting it. Eret tried to get rid of it twice by throwing it off of the ship, but Stormfly, thinking that he was playing 'fetch' flew after it and brought it back to his feet both the times.

After being captured by the Dragon Thief, who was unknown to him at the time was his own mother, and her dragons he initially tried to use it to pacify them but was unsuccessful, due to the sheer amount of dragons surrounding him. Following this, he primed Inferno with one of the Zippleback gas canisters that he kept on his boot and sprayed it around him in a circle before igniting it, succeeding in calming them before his mother interfered.

When he fought Drago Bludvist on Berk, he used Zippleback gas and ignited it as he flew past to knock him off his Bewilderbeast and, after making it safely to the ground, threw the inflamed Inferno to prevent Drago from reaching his bullhook, the tool he used to control his Bewilderbeast.

The Serpent's Heir
Hiccup uses Inferno to defend himself from some angry villagers. Hiccup then prepares to use it against Calder but due to being outnumbered, surrenders. Calder then takes Inferno from Hiccup and wields it throughout the rest of the novel. When he tries to attack Hiccup with it, the latter throws a canister of Zippleback gas at the blade, which explodes on impact, causing Calder to fall off a cliff and ultimately die. When the other Riders arrive at the scene, Eret picks up Inferno and gives it to Hiccup, warning him of its hotness.

Dragonvine
Hiccup will be seen using Inferno to chop his way through thick vines. He also uses it as a makeshift flaming arrow set in Hiccup's Shield as a cross-bow function to scare off Silkspanner dragons.

DreamWorks Dragons: To Berk and Beyond!
"As with most of my inventions, the Dragon Blade is always a work in progress. It still rattles and backfires from time to time, and I've been experimenting lately with using different types of dragon saliva and gas. For instance, I just learned that Scauldron gel burns even when wet. And Smothering Smokebreath gas makes for a pretty reliable smokescreen when we're about to make a hasty exit!"

- Hiccup

Hiccup relates that he is constantly updating his Dragon Blade and trying new things out on it.

Abilities

 * Monstrous Nightmare Saliva: By placing a small canister of Monstrous Nightmare saliva inside the hilt, Hiccup was able to coat the blade with the substance so that when ignited it produced replicated dragon fire, which convinced most - if not all - wild dragons that Hiccup was one of them. This ability could also be used to fight and intimidate humans.
 * Hideous Zippleback Gas: By placing a small canister of Hideous Zippleback gas inside the hilt, Hiccup was able to release the gas through the pommel and ignite it to create an explosion, in the event that Hiccup became surrounded by dragons, to prove to large groups of dragons simultaneously that he was 'one of their own'. This ability could also be used to fight or intimidate humans. The gas, using the sword, could also work like a blowtorch or a flamethrower.
 * Lighters: Inferno possessed two lighters to create sparks; each at opposite ends of the hilt as one was used to ignite the Nightmare saliva while the other was used to ignite the Hideous Zippleback gas.
 * Retractable Blade: The blade of Inferno was hollow to allow pieces of the blade to slide into each other before eventually sliding into the hilt, where it is soaked with Monstrous Nightmare Saliva.

Trivia

 * According to The Art of How to Train Your Dragon 2 and Race to the Edge episode "Tone Death", the sword is nicknamed Inferno.
 * Inferno is one of three tools used to influence or control dragons, along with Valka's Staff and Drago's Bullhook.
 * Given the blade's features, most notably its ability to retract into the hilt, Inferno may be a reference to the lightsaber, the signature weapon of the Jedi and the Sith from the Star Wars franchise.
 * A deleted scene shows that it is possible to use the saliva of dragons other than the Monstrous Nightmare for the blade, like the Stormcutter.
 * Though in that scene, it is shown that the Stormcutter's saliva produces an uncontrollable flame on the blade.
 * Inferno appeared in the game Dragons: Rise of Berk as a collectible in completing collections.
 * The Hideous Zippleback gas function is said to be inspired by the Dragon Eye, which emits stun gas as a security mechanism.
 * Unlike most dragons, the Small Shadows don't like the blade.
 * Hiccup's redesigned Inferno has many features influenced by Viggo's Fire Sword.