This shield is a multipurpose tool used by Hiccup Horrendous Haddock III.
In response to the increased danger from Alvin the Treacherous and his Outcasts, Hiccup created a special shield for his personal defense. At first it was a wooden shield in "Live and Let Fly", until Hiccup reinforced it with Meatlug's Gronckle Iron in "The Iron Gronckle". Hiccup then used his shield to defend himself and Toothless against Dagur on Dragon Island in "The Night and the Fury".
Dragons: Race to the Edge
Since "Imperfect Harmony", Hiccup continued to use the shield as a protection against wild dragons and Dragon Hunters.
The shield started out as a wooden disc with an iron rim and center hub. Hiccup also painted an image of Toothless on the face of the shield. The iron was then replaced with Gronckle Iron, which was also used to reinforce the front surface as shown in "The Iron Gronckle". Hiccup eventually added a wide leather strap so he could sling the shield onto his back, instead of having to hold it everywhere as shown in "A View to a Skrill, Part 2". It also appears that the strap allows him to secure it to the control cables on Toothless' right side, under the wing for travel storage.
In addition to deflecting incoming projectile and blows, the shield has some offensive capabilities built into it.
The center hub contains a retractable hook and cable which can be fired off like a grappling line, allowing Hiccup to entangle a foe or to hook something from a distance. It is shown that Hiccup's lack of strength can be used against him as when the line tried to pull Gobber in, his weight exceeded Hiccup's strength, causing Hiccup to get reeled in as shown in "Live and Let Fly". Hiccup used this feature while fighting Dagur the Deranged as shown in "The Night and the Fury".
The second offensive weapon is a hidden catapult on the rim. Although never seen in combat, it tends to trigger when anyone other than Hiccup picks it up, whacking them in the face. Hiccup used to tag several nadders with paint near Dragon Island as shown in "Appetite for Destruction".
It is later revealed that the Gronckle Iron doesn't just cover the shield, but was made into moving parts. This reinforcement made the shield an extremely sturdy platform to build on to, and allowed Hiccup to increase the shield's offensive capabilities by adding a crossbow conversion. This allows him to shoot arrows by hand, and other objects from a bowstring which acts as a high-powered slingshot. The center hub swings away and with the sides splitting apart, it creates tension for the bowstring, readying it for use as shown in "The Night and the Fury".
It's revealed that Hiccup has reused the idea of the Bola Launcher, which he had used to shoot Toothless down during a dragon raid. He can load one bola into the center hub and launch it at an enemy, like he did to the Alpha Speed Stinger as shown in "Frozen".
Gronckle Iron Plating
The first major modification of the shield was replacing the iron with Gronckle Iron, which was also used to plate the face. This made the shield lighter and stronger than before as shown in "The Iron Gronckle". It also makes the shield heat-resistant and can deflect flaming arrows as shown in "Maces and Talons, Parts One and Two, as well as throwing daggers and other projectiles. It can also be thrown like a boomerang to disarm an opponent as shown in "The Night and the Fury".
As the shield is coated with Gronckle Iron, this makes it very light and handy. Gronckle Iron is a very lustrous material and allows the shield to catch and reflect plenty of light. this light can be used to attract or repel dragons which are attracted or scared of light. The metal reflects light, which can drive away Whispering Deaths or attract the Screaming Death as shown in "Tunnel Vision".
In addition to that, Gronckle Iron appears to be a be a very heat resistant material, perfect for fending off wild dragons. it has been shown to resist against an Armorwing's flame, which is composed of oxygen and acetylene and reach soaring temperatures. It could hold up against a Night Fury's blast in "The Eel Effect", a Thundedrum's in "Imperfect Harmony", and an Armorwing's blast in "Snotlout Gets the Axe".
Since plating the shield with Gronckle Iron, Hiccup has discovered that using it against a Skrill is not advisable, as the metal will conduct lighting as shown in "A View to a Skrill, Part 2". It could not protect Hiccup and Toothless from a Flightmare's mist as shown in "Maces and Talons, Parts One and Two". Since the shield reflects light, it is also a liablity when stealth is needed and the reflection will give away his position.
- Being left-handed, Hiccup tends to carries the shield with his right hand. This allows him to fire it in crossbow mode with a natural stance. He has also been shown holding it with his left hand, as it's his weapon arm, when he expects to be bashing opponents with it.
- Though he didn't think much about the shield initially, Dagur the Deranged soon began wanting one after seeing its many innovations in "The Night and the Fury".
- Astrid admitted that the shield was pretty cool after seeing Hiccup use the crossbow to tag a Deadly Nadder with yellow paint in "Appetite for Destruction".
- Also in "Appetite for Destruction", Hiccup named the technique he used to defeat the Screaming Death the first time, the 'shiny shield trick'.
- Dagur refered to Hiccup's shield as being "ornate" in "The Night and the Fury, and he mentioned that his sister used to have one like it as well.
- It is currently unknown what became of the shield, due to its absence in How to Train Your Dragon 2.
- It is likely that Hiccup still has it, but just left it behind.