Great work. Could you check out my article on 5e design? I'd love your opinions. I've got a few more coming, and it might interest you from a design point of view.
CRACKING 5E PART ONE: RACES!
Sure thing, I'll take a look tomorrow.
Horrid Wilting: a 30 foot cube (three dimensions)
OK, so I'm going to disagree with you on a few points here. Let's go through point by point. Yes it's 3 dimensions, but 30' cube (15' diameter) is quite a small area for the level. That said, if the damage was concentrated enough that wouldn't be an issue.
within 150 feet (30 spaces in all directions from you),
I have no problem with the range, its the effect that I have a problem with
doing 10d8 necrotic (relatively few foes have immunity to necrotic, compared to something like Fire)
ALL undead and ALL constructs are immune, and that's before we even look at necrotic resistance or immunity. That's not good, that's actually quite terrible. Undead alone are probably making up more enemies than all the creatures immune to fire in most campaigns. The amount of creatures immune to this spell is one of the big problems it has.
Average damage would be, what, 45 on a failed save?
A Failed CON save by the way. This is going to be the 2nd worst save to target at high levels (Str being the worst, though few things target strength). Those high CR monsters tend to have high Con scores, which means more made saving throws. Average 22 damage on a successful save. You need to be 15th level to cast this spell. You can cast it a maximum of once per day. 22 or 45 damage on average. That's for your premiere 1/day spell. That's not good. Average HP of a CR 11 creature is just shy of 200. That's CR 11 - you will be facing lots of such creatures for even a run-of-the-mill encounter.
Group damage for 8th level spells should be 13d6 (average 39-ish?), so you're still doing more average damage than other 8th level area spells.
You are talking about using a Fireball (3rd level spell) with an 8th level slot. Generally speaking, using low level spells with high level slots is just a terrible idea. We really aren't comparing apples to apples.
How about we look at Chain Lightning. Chain Lightning is a 6th level spell. Let's cast it with a 6th level slot and compare to Horrid Wilting:
1) Range: 150' in both cases - winner: tie
2) Damage: 10d8 in both cases - winner: tie
3) Damage immunity: (Electricity immunity/resistance) vs (Necrotic Immunity/Resistance + All Undead + All Constructs): Winner Chain Lightning
4) Saving throw: Dexterity vs Consitution: Winner Chain Lightning
5) Area of effect: 30' cube vs 30' radius (both 3 dimensional) Winner Chain Lightning
Now Chain Lightning has a maximum number of targets it can hit, but on the other hand, there is no friendly fire. That's probably a wash.
Chain lightning wins hands down. This is a 6th level spell, and I rated it orange and said "it's probably not worth it", still, superior to the 8th level Horrid Wilting spell.
I should also mention though, that if you are especially interested in Blasting spells - The Wizard Guide that really lays it out the best isn't mine, it's this one:
Arrive on Time, A general guide to Wizardry. Check it out, it's well worth your time. I will note that he rated the spell better than I did (purple - his "so-so" rating) and pointed out that it's probably a good choice for Necromancers because of synergy with Grim Harvest.
It's a weird blasting spell, and arguably 8th level should be more interesting than blasting spells... but the math holds up. Plus you've got all that water now, so you could freeze it, shape it, etc, I suppose.
I'm not saying it's blue, but I would suggest better than ye stinky brown. Unless all damage spells at 8th level are auto-brown, this unusual fireball-type spell, with above-average damage, shouldn't be relegated to stinks-ville.
So a couple things:
1) I don't think the math holds up (as I pointed out above)
2) I don't think you actually get the water, I think it is gone. At least, that's the way I read it.
3) "Unless all blast spells at 8th level are auto-brown" It actually is pretty close to that. There really aren't any good blast options for your 8th level slot. Just look what Meteor Swarm is going to do with your 9th level slot. Shouldn't an 8th level blast be 1/2 as good? It's not even close to 1/2 as good.
But yeah, lower level would be more interesting, considering you could, I don't know, create a clone or have a demiplane with the same slot. Monte Cook had a 3e 9th level spell that aged characters instead of doing hp damage. THAT would be a great 8th level spell for 5e.
I should mention at this point that I rated Meteor Swarm green, and it's just a blast spell. However, unlike Horrid Wilting, the range, area and damage are appropriate for a spell of the level. IMO