Updated.
@DragonParadox Would these Mythic effects be reasonable?
Shadow of the Doom
Transmutation [Fire]
Level: Sorcerer 8
Components: V, S, M
Casting Time: 1 round
Range: Long (400 ft. + 40 ft./level)
Area: 20-ft. cube/level
Duration: 1 hour/level
Saving Throw: Fortitude partial; see text
Spell Resistance: No
The area you designate becomes a blistering hot environment, immediately raising the temperature by 3 temperature bands.
For example, if the temperature is moderate, it raises to severe heat.
If the new temperature is above the hot band, all surface water is turned to vapor and all earth and stone becomes igneous rock or volcanic soil (caster's choice) to a depth of 10 feet per caster level.
Air within the area becomes choked with volcanic ash (cuts visibility in half) while the rapid changes of pressure and heat cause a thunderstorm.
Living creatures caught within the area when the spell is cast instantly turn to stone (as per the Flesh to Stone spell). Creatures normally immune to the spell, such as Undead, instead count as having succeeded their saves and take damage accordingly.
If a creature successfully saves, Shadow of the Doom deals 1d6 points of fire damage per caster level. This damage has the Searing property.
Creatures entering the area after the spell has been cast do not take this damage; however, all creatures in the area are subject to the normal effects of the ash fall, heat and the thunderstorm for the duration of the spell.
Objects in the area, including those held by creatures, are instantly covered in a thin layer of ash, making them slippery.
When a creature uses such an item (a weapon, lockpicks, a potion, and so on), it must succeed on a DC 10 Dexterity check or it drops the item before it can be used.
Fire spells cast within the area gain a +1 caster level.
Multiple Shadows may be cast in the same area to increase the effects (raising the temperature by an additional 3 bands).
The caster my designate a safe area no bigger than a third of the total area. The spell doesn't affect that area. Alternatively, he may select up to CL targets to be immune to the effects if he succeeds on a Concentration check of 22 + Number of people he wishes to isolate from the spell.
Arcane Material Component: A piece of obsidian.
Mythic: Damage increases to 1d6+1 points per level on a failed saving throw and those affected suffer 1d6 points of Strength, Dexterity, and Constitution damage as their flesh partially transforms into obsidian. You cannot, however, designate creatures to be unaffected by a Mythic Shadow of the Doom spell.
-Augment (3rd Tier): You can expend two uses of Mythic power to increase the damage inflicted to 1d6+2 per level and the ability damage to 2d6.
-Augment (8th Tier): By expending ten uses of Mythic power, the range of Mythic Shadow of the Doom becomes Line of Sight and its area of effect changes to 1,000 foot radius per caster level plus Mythic tier.