This claim-stone, affectionately dubbed Namey Rock by the twins, stated that the Dragon's Edge belonged to the them, as they were the descendants of Magmar Thorston who dedicated the island to the Thorston clan.
Dragons: Race to the Edge
In "Reign of Fireworms", Tuffnut and Ruffnut found it on Dragon's Edge. When Hiccup, Astrid, Fishlegs and Snotlout found it, they first thought that the twins made a fake claim-stone, but Gobber determined that it was a real claim-stone. It was Discovered that it was Made by the Twins' long lost great uncle, Magmar Thorston. This caused the twins to rule the island horribly, until they were forced to share the island in order to save the island from the Fireworms who were burning the island to the ground. Afterwards, Meatlug ate the claim-stone.