There is a 1% chance of the Shaman Heartless dropping the relic without the use of Lucky Lucky, and the more Lucky Lucky abilities that are being used, the greater your chances are of finding rare items such as this.
The weapon was renamed to Spell's Relic (スプルズレリック Superuzu Rerikku?) in Kingdom Hearts II Final Mix. A slightly powered-up version, Spell's Relic+, can be found in Kingdom Hearts II Final Mix+, dropped by Necromancers with an equally low drop-rate.
In Kingdom Hearts II, the Shaman's Relic has a slightly bent handle that is brown at its base, tan in the middle, and khaki-colored at its tip. The head of the staff's base is a white vertebra, and the head itself is a dark blue skull with yellow, angular spirals for eyes. There is a dark blue vertebra just above the white one, with a light midnight blue spike on either side. It also sports two curled, light midnight blue horns on the sides of its head.
The Spell's Relic has a handle that is dark brown at the base, brown in the center, and a tan tip. The first vertebra is off-white, and the two spikes on the second one are now yellow. Its horns are violet.
In Kingdom Hearts II Final Mix, the Spell's Relic+ now has a handle that is light brown at its base, caramel-colored in its center, and pale yellow at its tip. The first vertebra is now silver and the spikes and horns on the skull are orange. The skull itself is predominantly black, though its face is dark red.