Fixing Blight

 Blight is a 4th level Necromancy spell available to Druids, Sorcerers, Warlocks, and Wizards. The spell is a single-target burst of necrotic damage that plant creatures (like a Shambling Mound or Treant) automatically fail against, and which will automatically kill nonmagical plants.

It's also deceptively bad.

Spellcasters, with the exception of the Warlock, are generally built to be not as good at single-target damage-dealing than martial classes. A Fighter or Barbarian is meant to be more effective in such situations, because spellcasters can A: have a lot of utility to shape the battlefield in their favor and B: have lots of options for dealing damage to groups of enemies.

Still, there are single-target damage spells. At higher levels, Disintegrate and Finger of Death pour out boatloads of damage - those 6th and 7th level spell slots remain precious resources even at level 20, but these spells, when the saving throw is failed, do 75 and 61.5 damage, respectively (why does the 7th level spell do less than the 6th level one? Because Finger of Death still does half damage on a successful save while Disintegrate is fully wasted if they dodge it).

If we compare the damage of these spells to AoE spells of the same level, Circle of Death and Otiluke's Freezing Sphere both do roughly 35 damage (Circle of Death does 36 on average,) so Disintegrate is going to be twice as effective against a single target. We could compare Finger of Death to Firestorm, the latter of which does 38.5 average damage, or Delayed Blast Fireball, which does vary in power, but if triggered immediately does 12d6 (which to be fair is the same as a 7th level Fireball) or a max of 22d6 (though how often are you going to let it cook for a full minute?) - so minimum average is 42.

Thus, to really get our bang for our buck, these AoE spells ought to hit two or ideally three or more targets to match the power of our single-target spells.

Going back down to Blight, though, the damage is 8d8, or an average of 36 (actually it's the same as Circle of Death). That's not an insignificant chunk of damage, but it's not much more than AoE spells we can get at this level. Just Fireball is 8d6, and when upcast to 4th level, that's 9d6, or 31.5 damage, which is not that far behind but potentially hitting several targets.

Now, is that fair, given that Fireball is intentionally overpowered to maintain its iconic status? (It probably could be 6d6 and still be a perfectly good but probably more balanced spell.) I suppose we could compare this with Ice Storm, a natively 4th level spell, which does 2d10+4d6 damage (a mix of bludgeoning and cold,) coming out to an average of 25.

The question, I think, is what ratio our AoE spells should have versus our single-target spells. In the case of Disintegrate versus Circle of Death, it's a little over twice the damage - though Disintegrate's "save for none" does effectively lower its overall damage-per-casting. Finger of Death, then, might be our better model, as most leveled spells are save-for-half. In this case, then, our average damage of 61.5 is roughly 1.6 times as much damage as Firestorm (or around 1.4 or 1.5 times Delayed Blast Fireball except in extreme cases).

Our ratio for Blight is actually pretty comparable to Ice Storm (though Ice Storm also creates difficult terrain) but is only about 1.1 times the damage of a 4th-level Fireball. It also targets Constitution, a saving throw that basically no creature has a negative to, and almost all have a positive modifier for.

So, what could we do to make this a more appealing spell? What could make this a fun spell to cast on a scary boss monster?

One thing I really like about both Disintegrate and Finger of Death is the solid chunk of guaranteed damage. If you had profoundly terrible luck (and in fairness, it would be a 1-in-16,777,216 chance) you could theoretically do only 8 damage with a Blight. But a Finger of Death, on a failed save, does at least 37 damage because it's not all die rolls, instead being 7d8+30.

I like this "flattening" of the damage. You could achieve similar damage to Finger of Death by rolling something like 18d6 (which gets you to 63,) but not only is that a lot of dice to roll, it also creates a lot of swinginess. That version could do anything from 18 damage to 108. The current version does 37-86.

It seems like this could be a general model for single-target spells.

So, then, how much damage should our new Blight do?

Well, we want to make AoE spells worth it, and the way things are currently balanced, it looks like this should be optimal as soon as there's a second target. So we want Blight to be less than twice the damage of a comparable-level AoE spell.

If Ice Storm does an average of 25 damage, and a 4th-level Fireball does 31.5, I think 50 seems like a reasonable target for our ideal damage (and this for sure is a buff). Now, how do we achieve that?

We need to pick what our flat damage should be - Finger of Death adds 30, while Disintegrate adds 40. I think we could go with 10 or 20, but I'm leaning more toward the later.

So, with 20 flat damage, we then want to do somewhere around 30 (maybe skewing lower) damage with dice. Given that the spell historically has used d8s, let's continue to use those. 6d8 does an average damage of 27, so this looks pretty good to me.

Thus, to make this spell feel really good, I think rather than 8d8, it should be doing 6d8+20.

Will this be utterly nasty against Plants? You betcha. But Plants are also one of the smallest creature categories in the game.

Naturally, some playtesting is required. 47 damage is a lot to be able to put out in a single spell at level 7, but I don't think it's completely outside the realm of reason.