and, so done!
First of all, the breath weapon format taken from the MM, for reference:
Breath Weapon (Su): 30-foot cone?, once every 1d4 rounds, damage (energy or effect), Reflex DC X half (or Fortitude DC X negates). The save DC is Constitution-based.
(see behir, chimera, dragons, dragon turtle, frost worm, iron golem, gorgon, hellhound, cryohydra and pyrohydra, mephits, winter wolf)
Do we want to incorporate this part? “it causes him 10 hp internal damage for every blast beyond the first.”
Note that the original description says “a 30' cone that extends at its widest point to 15 feet”, but I don’t know how that works in 3.5 for a Colossal dragon.
Should we put in a note about unattended objects, or should that be assumed by a DM anyway? “Nonmagical and magical objects must save vs. magical fire or be instantly burned to a crisp.”
“On the following round, anything or any creature within the path of Vore's breath weapon must make a saving throw again or be overcome by the lava that rushes down the mountainside” - OK, it does not specify how many rounds the lava lasts, so that will be up to our interpretation. All it says is that creatures take damage on the round following the breath, not whether the lava lingers beyond that second round.
“The lava inflicts 15d6+15 hp damage to anything in a 15' wide area within 100 feet of Vore's snout.” It took me a second to think of what this meant, but I think I get it now. since the breath weapon in 2E was 15-feet wide, essentially the lava would trail down the side of the mountain in a 15-foot “line” and go for 100 feet before stopping.
Perhaps, then, we could say the lava keeps moving until it reaches 100 feet. If it moves X feet per round, then the duration would be 100 divided by X.