The Forge is the loyalest source of upper level items in King's Road. To upgrade your gear give Magnus, the blacksmith, six items of the same rarity, his fee, and the requisite time. Your new item will be one tier rarer and its level will be "based on a random roll that is within a range of your current character's level." It is not possible to forge Artifact Items.
As Forging can take a considerable amount of time, it is recommended to have extra vault slots to store items waiting to be forged.