Immortals of Aveum

Once you have acquired a legendary sigil. You acquire one during the ‘morbane’ boss fight in the mission ‘colossal’. along with 4 legendary essence, approx 250 essence and 100k gold you can level it up at a forge. The game does not save immediately so you can fully upgrade the item then close the game after receiving the achievement to reuse the legendary essence for the other similar achievements. I do not know how plentiful legendary essence are but if it helps I have only found 8 at this point of the game so they appear pretty rare.

Note: if you upgrade a legendary sigil from DLC it doesn’t count.