Azura's Star is the reward for completing the Azura quest, it is a reusable soul gem useful for Mages with enchantments on their staffs and swords.
It is possible to trap all the souls of creatures in; however it cannot store NPCs souls in. Also, it cannot be turned into a black soul gem. In effect it is a reusable Grand Soul Gem and can help clear space in your inventory as you won't need that many more Soul Gems, if any at all.
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