To unlock The Midas Touch trophy in Assassin's Creed Odyssey, you will need to engrave a Legendary item (Gold) with a Legendary Effect (Gold).
Your first Legendary item is likely to be Nikolaos' Sword after completing chapter two of the game. Once you have a Legendary item, equip it in the inventory screen, and then scroll your cursor down to the hammer and chisel icon which contains your engravings.
Legendary Engravings have a gold border/background. Look through the list and you might have unlocked some already. If not, you can see what you will need to do in order to unlock it by hovering over it with your cursor.
Once you have a Legendary Engraving available, head to a blacksmith, select engrave, choose your sword or other Legendary Item and engrave it with the Legendary Effect.