It is called Amulet of Nabu by Dr. Fate.
The amulet gives the wearer vast magical abilities as resist any astral interference. It also houses a magical void that can hold people inside it. During Hector Hall's tenure as Fate, previous incarnations of Doctor Fate resided in the amulet to offer guidance when he needed it. For a time Mordru was imprisoned in the amulet as well as Nabu himself for a time.