What is the most damaging one shot spell?

[RUMBLETiGER]

So...how much xp is the population of Waterdeep/Greyhawk worth?

At 1 hour/level, and since the OP stated level 20 caster level if you dump this into the water supply you just might kill most of a city in 20 hours time. I don't know how much it would dilute into a water supply. I was thinking just soaking one person in it, but yeah.

instead of black lotus extract, I'd create one of the inhaled poisons. Although the damage per target is lower, a single dose of inhaled poison affects a 10ft cube, allowing you to hit more targets per dose (depending on size of the victims). This magnifies the possible damage on medium creatures by 4, and allows an area of slightly more than 300 x 300 ft to be covered in a 10ft high poison mist.

Minor Creation makes, "Unattended, nonmagical object of nonliving plant matter, up to 1 cu. ft./level" so consult Arsenic & Old Lace for ideas. The poisons able to be created by Minor Creation Are Listed Here. I picked Black Lotus Extract because the OP asked for most dame from 1 spell, and as far as I know that's the hardest hitting poison Minor Creation​ can make.

 

[Shemeska]

Scourge of the Horsemen (see BotD3) is pretty rough. Up to 20d6 acid damage and 1d4 negative levels to everything in a 30' radius burst.

 

[Stormonu]

Implosion - kills one creature per round (up to 4, regardless of its hit point total.

 

[ImperatorK]

Vengeful Gaze of God. 305d6.

 

[Nezkrul]

one thing about black lotus extract- the "extract" part; it is derived from the naturally occurring chemicals in the flowerS; not one flower, MANY MANY accumuluted samples of the chemicals. So minor creation can't make the poison for you, it is crafted from the flower; the spell can make the flowers for you though.

Delayed Blast Fireball is 20d6 at 20th, Polar Ray 20d6 to one target that can crit (no save), Isaac's Greater Missile Storm is 20d6 force damage (no save)

 

[Nytmare]

Vengeful Gaze of God. 305d6.

Don't forget to add in the 200d6 backlash! 505d6 total!

 

[PureGoldx58]

I wondered when Vengeful Gaze of God was going to be dropped. I'd say Miracle/Wish, because you can Miracle the moon to smash your opponent, or Miracle every damaging spell ever to go off into that enemy's face, etc.


Also, on the water I have used hurricanes created by Control Weather to destroy entire fleets of pirates. That is a more liberal use of a spell though.

 

[sheadunne]

Disintegrate.

Just pair it with Improved Critical (ray) and jack the save DC as high as possible. 80d6.

Add in Split Ray Metamagic and you got double the fun.

 

[Nezkrul]

searing, split-ray, empowered, twinned, energy admixture fire, fiery, maximized scorching ray is pretty biff

8 rays that deal 56+1/2(8d6+8) fire damage each and that ignore any fire resistance and still deal half damage to targets that are immune to fire

with the right build (NOT wizard 20), it ends up being around a 5th or 6th level spell slot

 

[nijineko]

i believe epic level spells hold the top slot for most damage. being that they are uncapped, and all that.