Stygian is a medium armor set. While you are Sneaking or Invisible, your damage done with Magicka Abilities is increased by 20%.
Stygian is part of the base game and drops in Coldharbour in the Overland content. You can also find it on Guild Traders. Each set item is bound on equip. You can acquire medium armor, jewelry and weapons for this set in the Draugr motif style.