Deluge of the Abyss calc from the first story Battle is an Art. This exact feat has to do with Herah firing off a fireball that, after exploding, left a sizable crater in its wake.
Feat[]
So I'm utilizing two bits of data here. The actual description within the story.
And just like that, an explosion of blue flames that reached higher than the treetops and engulfed what Herah figured to be just over 800 acres worth of forest.
BOOM!
The sound barrier broke as Herah launched herself into the flames and slammed into the ground. 'Herah’s impact kicked up a cloud of ash that was batted away with a single flap of her wings, which also cleared away most of the surrounding flames. With the air now cleared, Herah began searching for Moon.
The youngling now stood within a perfectly hemispherical crater. The entirety of it was just filled with fires of varying sizes and ash. The youngling paid little attention to the large plumes of smoke and steam that filled the air, her eyes trying and failing to find any sign of her opponent.
And a statement from a character.
"When you create as much destruction as my brother and I do on a regular basis, you learn how to gauge destruction in general very well. The crater we’re standing in has a surface area of 9712 acres and it’s around 6700 feet from its deepest spot to ground level."
Calculation[]
Using these two bits of information I'm able to determine that the radius is 6700ft and that the volume I need to find is that of a hemisphere. After converting the feet into meters and calculating the volume this is what I get:
6700ft=2042.16 m
((2/3) × 3.14 × 2042.163)= 17828207704.7m3=1.78282077047e+16 cm3
With that, I feel is safe to assume that vaporization is the proper destruction value due to the presence of steam and smoke and since only ash was the physical matter left.
1.78282077047e+16 x 25700= 4.8136161e+20(j)
4.8136161e+20/4.184= 1.1504819e+20/1018= 115.05 gigatons of TNT (I rounded to the nearest hundredths for this part)= Large Island level
EDIT: Recently looked over the calc and found that the crater size was incorrect so now I'm gonna just redo all the above the right way this time.
((2/3) × 3.14 × 2042.163)= 17828207704.7m3=1.7828207704e+14cm3
Once again, since steam and smoke are present and because fire was the medium used to sow this destruction I shall use vaporization.
1.7828207704e+14 x 25700= 4.5818494e+18(j)
4.5818494e+18/4.184=1.0950883e+18/10^18=1.1 gigaton of TNT (Rounded to the nearest hundreth's once again, just ended up with a single number in the tenths place because math)= High 7-A
Edit: Correction to my correction, Spine was kind enough to do the math within the comments so I see it fine to put this here
9712 acres = 3.9303e7 m^2
The surface area of an ellipsoid is approximately equal to 4*pi*(((a*b)^1.6+(a*c)^1.6+(b*c)^1.6)/3)^1/1.6 (relative error of 1.016%). Thus for a semi-ellipsoid it would be 2*pi*(((a*b)^1.6+(a*c)^1.6+(b*c)^1.6)/3)^1/1.6.
Now a=2042.16 m, and let's assume b and c are equal, and let them both be x.
2*pi*(((2042.16*x)^1.6+(2042.16*x)^1.6+(x^2)^1.6)/3)^(1/1.6) = 3.9303e7
x = roughly 2735.10
It is roughly hemispherical indeed, so it fits.
Volume of a semi-ellipsoid:
2/3*pi*2042.16*2735.10*2735.10*1e6*25700 = 8.222955147328846555e20 Joules, Large Island level
around 197 gigatons
Upgrade weirdly enough?