The Stormblade is the sword of Alvin the Treacherous.
The Stormblade originally belonged to Grimbeard the Ghastly, Legendary Pirate and King of the Wilderwest. Grimbeard buried the sword alongside a small fraction of his treasure in Skullion Island, leaving just a map in his coffin that hinted to its location. In addition the map stated his 'true heir' would be the one to find his hidden treasure.
Because Fireworm found the sword, it was believed by the Hairy Hooligans that Snotface Snotlout had fulfilled the prophecy and he was the true heir of Grimbeard the Ghastly. However that ws just a decoy and the real treasure was found by Hiccup Horrendous Haddock III.
Later, Hiccup finds Grimbeard's true will in the pommel of his sword, Endeavour, which reveals that Endeavour was Grimbeard's favorite, as "the Stormblade always lunged a little to the left."
Alvin keeps the Stormblade with him at all times, and has crafted an attachment that allows him to fix the sword to the stump of his missing hand.
The Stormblade is a large, sharp, shiny sword. The sword itself, the hilt, and the scabbard are all very ornately decorated and very rich-looking.
- "Snotlout had already reached in and drawn out a truly magnificent sword, the scabbard richly decorated with dragons, skulls and the waves of an angry sea.
THAT was a sword fit for a Pirate King. It made a soft hiss of a serpent as Snotlout gently drew it out of the scabbard, and as the sunlight glinted on the still-bright, cruel blade, you could see how bitingly sharp it was, even after all these years underground.
On the handle was a furious portrait of Thor the Thunderer with a tangled seaweedy beard, and across the blade was a zigzag lightning pattern in a lighter silver."
- ―Book 2