Thank you for your answer! Then, I have two more questions:
1) Does soldier's luck actually affect the chance of Hellfire triggering, or not?
2) Regarding the damage of the "Exploding Corpses" ability, here's a quote from the game: "When an enemy unit is absorbed, a fiery explosion occurs, equivalent to casting the Fireball spell, but with magic skill = 1 if an unupgraded unit is absorbed, and with magic skill = 2 if an upgraded unit is absorbed."
So, does this mean that when an upgraded corpse is absorbed, a double Fireball is cast? Or is it simply that a Fireball with 20 + 20*2 = 60 damage is cast for the upgraded unit, and a Fireball with 20 + 20*1 = 40 damage is cast for the unupgraded unit?
If it's the second option, then the ability isn't that useful. And by the way, does it affect your own units?
2) The damage from "Exploding Corpses" is indeed small – it slightly helps with the "Phoenix Robe";
however, the benefit may also be that the explosion destroys the low-level units acting as shields. Of course, you need to be careful, as the damage is also dealt to your own creatures. When calculating the damage, you should use the formulas from the manual (the calculations for "Fireball" from Chaos Magic)