The title says it all.
The spell would be something along the lines of:
"As Disintegrate, but as a line shape, and can effect multiple objects and creatures. It must make a separate attack roll against each target. The line has a length equal to the range of a Disintegrate spell. This means that the length is effected by the caster's CL. If the spell fails to disintegrate something within the path of the line that it hit, it does not effect any objects or creatures further away from the caster than the target that the spell failed to disintegrate. A creature forced to make a saving throw against this spell can elect to take a -4 to the save, if it has more than 5 HD. If it elects to do so, the spell acts as if it had failed to disintegrate the creature, even if it did. This effectively allows creatures to shield the area behind them from the effects of this spell."
Personally, I think this is an 8th level spell.
Does it still require a ranged touch attack and a Fort save? I'm assuming yes, though it seems odd on an area spell.
My first thought is to compare it to disintegrate + widen spell. Which comes to 9th level. If you were to replace the ranged touch attack with a Reflex save I could see 8th maybe, saves are less reliable offence than ranged touch attacks.
Since the spell is stopped the moment it fails to disintegrate something (and TBH, disintegrate has a tough time because Fort saves tend to be the highest enemy saves) it likely won't affect more than one relevant target. It has incredible BBEG/narrative potential when used to slay a whole line of townsfolk or mid-level adventurers, but in that sense so does a maximized lightning bolt. Against creatures of a similar CR, one of the following will happen to the first target you hit:
Seems like that last option will happen fairly rarely based on my experience with using the disintegrate spell.
Its not clear what the object-based limitations are though. How much volume can this version drill through?
Assuming the volume is still limited, I think 8th level is fine for this spell.
Does it still require a ranged touch attack and a Fort save?
My first thought is to compare it to disintegrate + widen spell. Which comes to 9th level. If you were to replace the ranged touch attack with a Reflex save I could see 8th maybe, saves are less reliable offence than ranged touch attacks.
It still requires a touch attack and a Fort save. Maybe I should change it to reflex.
You miss your attack roll on a nat 1 or because the creature has high touch AC, or because it has mirror images, concealment, etc. On a miss, it ends because it failed to disintegrate something.
The ending effect only happens if you hit something, and it fails to disintegrate. I should probably include something about how a creature can choose to be struck by it if an effected square is within it's reach or space.
Its not clear what the object-based limitations are though. How much volume can this version drill through?
The original idea for this spell isn't that of a pure combat spell. The primary use for this spell is the ability to put a 150-ft+ hole into the side of a mountain instantaneously. This is something that is used to destroy castles, sink ships, cause floods or as an escape method. (AKA, have feather fall prepared and aim it immediately below you, with the knowledge of caves below. Would be a good spell to know deep underground.)
It's also a narrative spell, like you said. I want PCs and NPCs to actually sweat over whether to take that -4 to hit, especially if you're fighting in a city, a Volcano or something. Maybe the sacrifice mechanic needs some work?
I toyed with the idea of the beam staying active over the course of a round or two. You could move the beam from side to side as a full round action, AKA you can slice the top off a mountain. But I think that would be a 9th level spell. Maybe Disintegration Beam II.