Skills for an Inferno hero.

Why didn't anyone mention Light magic? Sure, it becomes available for study quite late, but when playing against Inferno and without the possibility of a blitzkrieg, it can be very useful. Inferno always tries to kill demons in a few stages, i.e., stage number 1: killing Imps, stage number 2 (code name: Operation Weasel Climax): the end of Inferno. And from Light magic, we need Resurrection and Stone Skin, i.e., we resurrect the Imps and apply Stone Skin, although because Resurrection appears quite rarely in Inferno castles, this magic becomes a bit of a "joke."

Imps often manage to move around before they are killed... so all subsequent "Operations" are canceled.
Light magic is only useful for demons because of "Blessing," as the damage range is very large. But because of this, taking up slots and spending levels... in my opinion, it's not worth it. Expert Mass Haste can be easily obtained through the Speed magic spell, and the hero's remaining moves should be spent on Dark magic spells - Slow, Weakness, Curse, Dispel, Blind, Hypnotize... That's the Dark magic that should be upgraded, especially since it is the strongest of all magic schools in Heroes 5, and demons are specifically designed for it.