War Maiden is a light armor set. The 5-piece set bonus adds 400 Spell Damage to your Magic Damage abilities.
This makes it a popular set for Magicka Templars, Nightblades and Wardens, who do a lot of Magic Damage.
War Maiden is part of the Morrowind Chapter/DLC and drops in Vvardenfell 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 Telvanni motif style.