I agree with Sunic that in general it's still pretty deadly, but it does pull it back some. If we drop it to 1d3 per level, a typical 6th level SoD will do 6d3 Con damage (average 12, with a range of 6 to 18). It could kill you, but it's less likely to do so, espescially at that level. It also helps to keep it a mook-killer. Even a 9th level spell would be dealing 18 Con damage on average.
Which means you lose 6 HP per level and any follow up is going to kill you.
My proposed solution, based on this link
here, for the problem of SoDs is to convert them to Con damage. As Robby has stated above, this is quite powerful, and Sunic expressed concerns over such. I wanna put this to the statistical test to see what happens against level-equivalent opposition in terms of damage dealt as a result.
Now, our premises are as follows: our caster has casting stat set at 22 (a reasonable expectation), for a total DC of 24 (10+6 [spell level] +6 [stat]+ 2 for Spell Focus), and we'll be pitching this against monsters of CR 12. The spell offers a Fort save for half.
My hypothesis is that, while significant, this will not be nearly as powerful as Sunic claims, being a solid, but not overpowering, option.
Abyssal Greater BasiliskRoll needed to pass the save: 6 or more
Damage on a failed save (average): 108 (57% of hp)
Damage on a passed save (average): 54 (28.5% of hp)
Conclusion: This is not unreasonable, given that the monster will pass 75% of the time.
Adult Brass DragonRoll needed to pass the save: 9 or more
Damage on a failed save (average): 114 (57% of hp)
Damage on a passed save (average): 57 (28.5% of hp)
Conclusion: Given that dragons are supposedly under-CRed, this seems a bit odd comparing to the last entry. However, the dragon still passes more often than it fails, and even if it does fail, it is not an unreasonable damage amount.
Young Adult Bronze DragonRoll needed to pass the save: 9 or more
Damage on a failed save (average): 108 (57% of hp)
Damage on a passed save (average): 54 (28.5% of hp)
Conclusion: See above.
Eleven-Headed Cryohydra/PyrohydraRoll needed to pass the save: 12 or more
Damage on a failed save (average): 66 (56% of hp)
Damage on a passed save (average): 33 (28% of hp)
Conclusion: Although it fails more often, the percentages it posts show that this is still alright compared to the other examples.
Frost WormRoll needed to pass the save: 10 or more
Damage on a failed save (average): 84 (57% of hp)
Damage on a passed save (average): 42 (28.5% of hp)
Conclusion: This thing is only slightly worse-off than dragons. Now that's saying something.
KrakenRoll needed to pass the save: 3 or more
Damage on a failed save (average): 120 (41% of hp)
Damage on a passed save: 60 (20.5% of hp)
Conclusion: This posts BETTER numbers than the dragons. Now THAT'S saying something.
LeonalRoll needed to pass the save: 11 or more
Damage on a failed save (average): 72 (63% of hp)
Damage on a passed save (average): 36 (31.5% of hp)
Conclusion: Now, this is reason to worry, as that leaves it perilously close to death, as described by Sunic.
Collossal Monstrous ScorpionRoll needed to pass the save: Anything but a 1
Damage on a failed save (average): 240 (80% of hp)
Damage on a passed save (average): 120 (40% of hp)
Conclusion: The damage is high percentage-wise, but in actual fact, almost every time, this will do LESS than it will against other monsters of its CR.
Elder Black PuddingRoll needed to pass the save: 9 or more
Damage on a failed save (average): 120 (41% of hp)
Damage on a passed save (average): 60 (20.5% of hp)
Conclusion: Percentage-wise, only the scorpion gets a better deal.
Purple WormRoll needed to pass the save: 7 or more
Damage on a failed save (average): 96 (48% of hp)
Damage on a passed save (average): 48 (24% of hp)
Conclusion: Some very good numbers.
RoperRoll needed to pass the save: 14 or more
Damage on a failed save (average): 60 (71% of hp)
Damage on a passed save (average): 30 (35.5% of hp)
Conclusion: This, along with the leonal, is the only case for concern.
Mature Adult White DragonRoll needed to pass the save: 7 or more
Damage on a failed save (average): 126 (52% of hp)
Damage on a passed save (average): 63 (26% of hp)
Conclusion: Nothing much of note here.
As can be seen, the range of damage on a failed save (worst-case scenario) here is anywhere from 60 to 240 (damage-wise) or from 41% to 80% of total hp. However, if we make those values cumulative with save probabilities, the gap narrows substantially, to 49.5 and 126, and 28.7% and 42% of hp. I do not believe that these are unacceptable values, especially if the spell is single-target and subject to resistance or immunity to death effects.
If I have made any mathematical errors, please point them out to me, and I will be happy to correct them.