The Armor Master set is part of the Imperial City DLC. The crafting station is located in the Memorial District of Imperial City. You can craft any armor, jewelry or weapon with this set in any motif style you know, as long as you have 9 traits researched. Crafted pieces can be traded to other players. Each set item becomes bound on equip.
If you need more information about the Imperial City, make sure to check out the full Imperial City Guide