Less a cursed item than a difficult and dangerous one to use, this will appear to be a carefully carved piece of flint, complete with runic characters, and a thin piece of steel, also bearing runes. They will come in a small wooden box with matching symbols.
Upon inspection, these two items, which can be considered a single one for these purposes, will radiate strong magic. If the enchantment isn't invoked, this can be used as a regular flint and steel, flicking small sparks with which ordinary fires can be started. If the magical phrasing written on the flint, steel, and box is deciphered and spoken, however, the sparks become Fireballs (like the spell) doing 6d6 HP of damage! Up to three of these can be cast per day with this item, with a range of up to about 450 feet.
The curse, as such, is that any time it is used to start an ordinary fire, there's a large chance that one of these Fireballs will be conjured by accident, whether the phrase was invoked or not, and regardless of how many were already cast that day! In this case, the Fireball will go off more-or-less in the user's face. The chance of this occurring is 50% minus 05% per level of the user, down to a minimum chance of 05%.