3. Azura's Star/Black Star
What it does: Azura's Star can trap white souls, which can be used for enchanting and for recharging weapons, while The Black Star can trap both black and white souls, up to the Grand level.
Where to find it: Shrine of Azura, to the south of Winterhold
How to get it: Get to the Shrine up in the mountain regions, where Aranea will proclaim you Azura's new champion and tell you to talk to an Elf in the Frozen Hearth Inn in Winterhold. Once there, you will speak to Nelacar the Elf, via Dagur, and then will have to travel to Ilinalta's Deep to retrieve Azura's Star from Malyn Varen. Once retrieved, you face the decision as to who you return the Star to for repair - Aranea (Azura's Star) or Nelacar (Black Star) - and then must enter it to destroy the evil souls within.