
Guess I should read up on the rules a bit more before I try to be a wise-arse. Still, there is a problem with Leadership and Additional Participants in Rituals.
How about this one (an adaptation of an official spell):
Rain Of Fire
Evocation [Fire]
Spellcraft DC: 50 => 36
Components: V, S
Casting Time: 10 minutes
Range: 0 ft.
Area: 4-mile-radius emanation
Duration: 20 hours
Saving Throw: Reflex negates (see text)
Spell Resistance: Yes
To Develop: 324,000 gp; 7 days; 12,960 XP. Seeds: energy (fire) (DC 19), energy (weather) (DC 19). Factor: change rain to wisps of flame (ad hoc +12 DC). Increase area by 100% +4
Mitigating: +9min casting time (-18 DC).
This spell summons a swirling thunderstorm that rains fire rather than raindrops down on the character and everything within a two-mile radius of him or her. Everything caught unprotected or unsheltered in the flaming deluge takes 1 point of fire damage each round. A successful Reflex save results in no damage, but the save must be repeated each round. Unless the ground is exceedingly damp, all vegetation is eventually blackened and destroyed, leaving behind a barren wasteland similar to the aftermath of a grass or forest fire. The fiery storm is stationary and persists even if the caster leaves.
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Pretty nasty. If the area was increased even more (and that's not very expensive) you could destroy entire countries with this kind of spell.