Once again, I submit for your general use a spell for Jim Raggi's Weird Magic System.
If you don't have said system, just make this a 1st level magic-user spell.
Fire is the great destroyer. Fire exists only to consume. While humans have found ways to tame it for their own use, fire is always looking for a way to escape, spread, devour.
Sorcerers have long been associated with spells that involve fire. Spells such as burning hands and fireball are commonly known, and rightly feared, by those who are familiar with the ways of wizards. However, there are some practitioners of the arts arcane who are not content to simply immolate their enemies, they leave them physically unharmed but seek to burn down their entire lives around them. Fire is one of fate's favorite weapons, and by means of this spell a caster can wield fire in a far more precise fashion than simply scorching the opposition.
In order to cast this spell, the magic-user must have been legitimately wronged by the target. What that entails is a discussion between the GM and the player, but generally the target must have caused the caster some form of harm or loss, intentionally and unprovoked. If these criteria are not met, the spell simply fails to function.
If successful, the target's entire life burns down around them. Their home, business, lands, all will catch fire within the next month, one at a time, and be destroyed in a conflagration that will happen as a contrivance of circumstances. The target will never, ever be harmed by these fires, even if the fire must go to miraculous lengths to spare them. Their loved ones, friends, prized possessions, and assets, however, will certainly be consumed. Fate claims the things that matter most to the target.
There are two ways to stop the spell before it runs its course (a 30 day process): First, the target can make reparations to the caster, which the caster must deem fit and accept. This ends the spell immediately. Secondly, the target must get a third party to cast Wish or similarly powerful reality-altering spell. In either case, stopping the spell does not restore the things burned to nothing by it. Nothing can ever restore those losses.
There is one terrible price to casting this spell: something, or someone, precious to the caster will erupt into flames after the 30 days this spell takes to run its course. There is no way to predict or prevent this, and no way to restore the loss. Even revenge has its price.
The things burned by this spell are completely immolated: only ash remains.
If the caster should fail to cast the spell correctly, consult the table below. At the DM's option, a caster who attempts to use this spell against someone who has not wronged them will force the caster to roll on this chart as well.
1- The spell works normally, but the caster will be forever haunted by burning ghosts of anyone killed in this spell. If for some reason nobody dies in the fires, the caster will be plagued with nightmares of the things they love burning down around them.
2- The spell works normally, but at its conclusion, the caster does not lose anything- they are immolated themselves. There is no way to prevent this, and the caster can never be raised.
3- As above, but the caster becomes a fire elemental. They are an NPC monster under control of the GM. This transformation is permanent and irreversible.
4- The spell works, but instead of losing something, the caster is permanently under the effects of the Volcanic Slut spell from VAM.
5- The spell works normally. In addition, the caster's head catches aflame. Even if extinguished, the caster will suffer hideous burn scars that reduce Charisma by 1d4 permanently. A character reduced to 0 Charisma no longer has a face. Any magic-user who looks upon the caster will know that these scars came from misusing this spell.
6- The spell works as intended, but the caster forever loses the gifts of fire. No attempt to start a fire will ever be successful. Torches and lanterns she lays her hands on will extinguish. She cannot use any spells that involve fire, including this one, ever again.
7+ Use the general spell failure table in the Weird Magic System.