Hiccup's Shield

In response to the increased danger from Alvin the Treacherous and his Outcasts, Hiccup created a special shield for his personal defense in Live and Let Fly.

Appearance
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. By A View to a Skrill Part II, a wide leather strap had been added so Hiccup could sling the shield onto his back, instead of having to hold it everywhere. 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.

Abilities
In addition to deflecting incoming projectile and blows, the shield has some offensive capabilities built into it.

Grappling Line
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. Hiccup used this feature while fighting Dagur the Deranged in The Night and the Fury.

Catapult
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.

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. It also makes the shield heat-resistant and can deflect flaming arrows, as well as throwing daggers and other projectiles. It can also be thrown like a boomerang to disarm an opponent. The metal reflects light, which can drive away Whispering Deaths or attract the Screaming Death.

Crossbow Mode
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.

Bola Launcher
In Frozen, 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.

Weaknesses
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.

Since the shield reflects light, it is also a liablity when stealth is needed and the reflection will give away his position.

Trivia

 * 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 Appetite for Destruction, Astrid admitted that the shield was pretty cool after seeing Hiccup use the crossbow to tag a Deadly Nadder with yellow paint.
 * 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'.