Shadow Dancer's Raiment is a light armor set. The 5-piece set bonus allows you to ignore the Movement Speed penalty of Sneak.
Shadow Dancer's Raiment is part of the base game and drops in Greenshade in the Overland content. You can also find it on Guild Traders. Each set item is bound on equip. You can acquire light armor, jewelry and weapons for this set in the Thieves Guild motif style.