What is the most powerful spell?

Solution 1:

Looking at the wording available for the Heck Hound spell:

Deals 130 Fire Damage per Pip over 3 Rounds (accuracy 75%)

It can be seen that the damage this spell can give is dependent on how many rounds you manage to stay fighting, as 1 more Pip is added for every round you stay fighting.

Thus, the only way to compare this spell in regards to damage is by comparing it to other spells that use the same number of Pips.

So, for the number of pips, this is the total average (Damage * Pips * Accuracy * 3 rounds) damage done by Heck Hound:

1   -   292.5
2   -   585
3   -   877.5
4   -   1170
5   -   1462.5
6   -   1755
7   -   2047.5
8   -   2340
9   -   2632.5
10  -   2925

We can see here that the spell's damage is quite high. Let's also assume that there is only one enemy, so that spells that target all enemies can be considered fairly.

Running down all math, I was able to produce an Excel file detailing all spells' damage. By analyzing the data one can check that Heck Hound is indeed the most powerful (in terms of damage) spell, except for the 8 Pips case, where King Artorius (Myth) alone gains a small advantage.

The Judgement spell from the Balance School has similar mechanics to this spell, but falls a lot short, due to the round bonus.

Solution 2:

If multiple I would say Call of Kuruhulu with colossal. It gives you 1k base damage. With blades against 4 people yout get health back instantly while crippling all. Alone I would probably say Judgement.