The Stormblade is the sword of Alvin the Treacherous.
History
The Stormblade originally belonged to Grimbeard the Ghastly, Legendary Pirate and King of the Vikings, but was found by Alvin on the Isle of the Skullions. Because the Stormblade was legendary, it was believed by the Hairy Hooligans that Alvin had fulfilled the prophecy that only Grimbeard's "true heir" would find his sword.
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.