Hephaistos's Workshop is a special vendor location that can be found within the Myson Cave in the Mallis region.
It can be easily reached from the Ruins of Artemis fast travel point in the same region.
To aid you on your journey, for a small fee, Hephaistos offers the following services:
Engrave allows you to enhance your weapons.
Hephaistos does not offer any additional benefits to the engraving services offered by regular blacksmiths.
Learn allows you to view all existing engravings in the game and purchase them with Drachmae.
Upgrade allows you to enhance your weapons with engravings of Tiers 6 and 7. 
It can be easily reached from the Ruins of Artemis fast travel point in the same region.
To aid you on your journey, for a small fee, Hephaistos offers the following services:
Engrave allows you to enhance your weapons.
Hephaistos does not offer any additional benefits to the engraving services offered by regular blacksmiths.
Learn allows you to view all existing engravings in the game and purchase them with Drachmae.
Upgrade allows you to enhance your weapons with engravings of Tiers 6 and 7.
Do not hesitate to let us know if you have any questions!