Azura's Star is a boon given to a champion by the goddess Azura.
A reusable soul gem, Azura's Star is filled with a soul and emptied thereof by the usual method. The messages, "You failed to make an enchanted item" and then, "Your soul gem broke" are always given when enchanting fails, even when using Azura's Star, which can be reused an unlimited number of times and never breaks.
Some quests will require the Nerevarine to give a soul gem, and in at least one of them, the Star will be taken if it is not dropped first. The Ghostgate portion of the "Pilgrimages of the Seven Graces" quest is an example of one that removes a soul gem even if Azura's Star is in inventory. The construction of the Telvanni Stronghold is another example of a quest that will take the Star (in this case, if it is filled with a powerful soul, the code's check for the soul bypasses the check for the Star).