The items and engravings required should unlock through natural progression of the game. The first Legendary Item you will unlock likely be Nikolaos' Sword
after completing Episode 2 of the game.
To get the achievement, you will need a legendary engraving for melee weapons, and the easiest ones to unlock early on are probably +20% Damage with Sparta Kick Ability
or +20% Damage with Spreadshot Ability
(for defeating the Patricidal Mercenary). Many will also be unlocked through natural progression of the game.
With both of these, head to a blacksmith and upgrade your weapon using the engraving to unlock this.