A necklace of fireballs was a series of enchanted beads made to explode on impact, creating a fireball Spell. Typically, the necklace's chain was magical, preventing the beads from detonating unless detached. When pulled, the beads could be thrown, and when they reached the end of their trajectory, they detonated in a massive fireball.
Multiple beads, or even the whole necklace, could be thrown at a single time, increasing the concussive force of the fireball. Once all the beads were used, the necklace was rendered non-magical.