The Black Star (Item)

is the quest reward for The Black Star, and is given to you if you choose Nelicor to repair it. It is said it will only house black souls (i.e. Humanoids) but it has been reported it will store any level of soul.

This is a Daedric Artifact, and can be used to recharge weapons or enchant with unlimited times without being used up.

Soul Levels of Creatures
At level 20, humanoids such as Conjurers, Druegar Wraiths, and basically anything that does not die in one hit - will fill the with a Grand Soul.