The Stormblade originally belonged to Grimbeard the Ghastly, Legendary Pirate and King of the Vikings. Grimbeard buried the sword alongside a small fraction of histreasure 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 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.