First, an undamaged skull must be cleaned, inside and out. The jaws also need to be affixed to the skull with metal wire.
The skull is placed on a very hot fire. For three hours, the mage must chant to encourage the skull to take the fire in, then for another three she must convince it to shrink until it is the size of a pine cone. Both of these stages require a saving throw or intelligence attribute check to proceed. If they fail, the skull shatters.
At the conclusion of this chanting, the soul is pulled back to the skull, which then speaks with the mage, who has but a few minutes to strike a deal. In return for some favor, the soul will hold the fire in the skull. Common favors include acts of vengeance, contrition, the delivery of a message or just letting the skull see the sun set over its homeland one last time.
The favor must be accomplished within the year, but once it is, the skull becomes a fireball grenade. When the mage says hold the skull and reminds it of the favor done, the fireball automatically hits and does maximum damage at one level higher than the mage’s casting level. These can be given to others to use, but they must know the favor and speak it aloud to throw (this is the ‘pin’).
If the mage screws up the favor, or does not complete it within the year, or dies, the skull goes mad, flying around breathing fire (as a young dragon) for one turn, targeting the mage and then any random bystanders.
These are often worn as part of a necklace, belt, bone bikini or other ridiculous fantasy getup.
Note: This is a first level spell with a casting time as mentioned above. One need not know the fireball spell to make one.