Gift from Lucatiel of Mirrah in Aldia's Keep if she survived three of her boss fights as a summon. Alternatively, Lucatiel can be killed in any location and her set bought off Merchant Hag Melentia for 18,000 souls.
Lucatiel's Set has an interesting blend of protection; the set provides absolutely no Poise, but it boasts decent physical defenses, durability, excellent elemental defenses, and good resistances.
While it does not provide any Poise, it provides excellent elemental defenses and a high resistance to every auxiliary effect when the set is fully-reinforced.
It upgrades with regular titanite.