The Black Star is one possible reward for completing the quest “The Black Star.” It is the alternative to Azura’s Star, a Daedric artefact of the Daedric Prince Azura. The Black Star counts toward the “Oblivion Walker” achievement or trophy.
The Black Star is a reusable black soul gem. As such, it is able to store black souls, which are always of the Grand level, like other soul gems, it can only hold one soul at a time. Unlike other soul gems, the Black Star is not destroyed after its contents are used – it is simply emptied and can be filled with another trapped soul.
While Nelacar claims the Black Star can house only black souls, it can actually hold souls of any type, like standard black soul gems.
The Black Star is recommended over Azura’s Star for players who want to develop the Enchanting Skill.
The Black Star is only featured in Skyrim and not any other Elder Scroll Games so far.