This fork has 5 charges. While holding it, you can use an action to expend 1 or more of its charges to cast the call lightning spell (save DC 15) from it. For 1 charge, you cast the 3rd-level version of the spell. You can increase the spell slot level by one for each additional charge you expend.
The fork regains 1d4 + 1 expended charges daily at dawn. If you expend the fork's last charge, roll a d20. On a 1, the fork shatters into pieces, is destroyed, concentration is lost, and you take 3d10 lightning damage on a failed Dexterity save (DC 15), or half on a successful one.
When concentration fails, the storm will continue until the duration of the spell is met. If this occurs, any creature within the 60-foot radius of where the spell was first cast at the start of their turn must make a Dexterity save (DC 15 + each additional charge expended). Damage received is based on the call lightning and the number of charges used.
The spell can be re-tuned by striking the fork again expending another charge and an action.
e.g. (base 3d10) + (1d10 * each additional charge expended) + (1d10, if there is an existing storm)