Made of Valyrian steel, Longclaw is the familial weapon of House Mormont. When Lord Jeor Mormont retired from his lordship to command the Night's Watch, he passed the sword to his son, Jorah Mormont, whose act of selling poachers into slavery disgraced the family and placed him in exile whereupon the sword returned to his father.
In the first season, Lord Jeor gave Longclaw to Jon Snow as a reward for saving his life from a wight. Jeor even remade the handle to replace the bear of House Mormont with the direwolf of House Stark.
During Jon Snow's expedition beyond the Wall in season seven, Jon offers to give Longclaw back to Jorah Mormont now that they are allies along with Daenerys. Jorah, however, releases all ancestral claims to the weapon and encourages Jon to wield the sword in good faith. In this respect, Longclaw replaces Ice as the new ancestral weapon of House Stark.
As the mission continues, Jon kills a White Walker with Longclaw and discovers that once a walker is destroyed, all the wights turned by that walker's hand die as well.