The spell works just like any other damage-dealing area spell. In a way similar to Ice Storm, since part of the damage is impact and part is elemental.
Perhaps MacMathan is confused because the spell description does not match the spell synopsis.
The spell synopsis states:
Eruption deals 5d6/level damage to all in area
Yet in the actual spell text it reads:
......creatures and objects within the area take 7d6 points of damage; the damage is 4d6 impact damage and 3d6 fire.
So which is it? Obviously the spell synopsis is in error when it says 5d6/LEVEL. It must have meant just 5d6. Yet this in conflict with the spell description which says the spell does 7d6.
Another question arises from the fact that this spell grants no saving throw yet in the spell description the following can be found:
Those that fail the saving throw are knocked prone.
So if there is no saving throw does that mean everything in the area is automatically knocked prone or does it mean that this spell is supposed to have a saving throw?
In situations like this, you go with the actual spell description, not the short form. There are also errors like this in other WotC books, especially OA.
As for the area, it's basically an army- or town-destroying spell. The damage doesn't scale by level, unlike most other damaging spells, so against small numbers of big dudes, it'll be of limited effectiveness.
The damage isn't what concerns me really, its the knocked prone thing. Being able to knock everything in the area prone with no save is a huge ability.
What do you guys think? Should it give a reflex save to avoid the knocked prone part? Further if this spell is to be kept in line with ice storm shouldn't the damage be 5d6 no save?
If it is left at 7d6 no save then why ever take ice storm?(I know there would be a very few reasons but on the whole this spell would overshadow icestorm if left as is.)