Eyes of Mara is a craftable set. The 5-piece bonus reduces the Magicka cost of Restoration Staff abilities by 12%.
Eyes of Mara is part of the base game. The crafting station is located in Eyevea. You will need to complete the Mages Guild questline in order to unlock access to it. You can craft any armor, jewelry or weapon of this set in any motif style you know, as long as you have 8 traits researched (per item). You can trade crafted pieces to other players. Each set item becomes bound on equip.