Posts from RTF is a global modification that replaces all towns in the game.

I'm reworking the soulless one; now it will attack with a fiery wave, and its health will be more reasonable.

There's also a mod that replaces Necromancers with Fallen Angels, Stone Giants, Black Orcs + a few Necromancers, and I have a mod that replaces Demons with Elementals + Dark Gnomes, but they only work together.

It's impossible to add new spells to the game or create duplicates. However, there are three spells that might be reworked:
1) Hellfire Wave of the Master (fire element)
2) Diabolic Chain of Biara (air element)
3) Uber Meteor Shower (earth element) - this spell wasn't really used in the game (unlike the first two).

Besides that, there are various prayers, banishments, and battle cries. There are also unit abilities (I wrote about them above) - in short, we need to try to fit these into magical schools or at least tie them to perks.

Is that the one with the "storm strike"?

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Do you happen to have a mod that replaces the Necromancers?

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Many have already been placed in Erathia, from Moon Elves to cyborg demons.

the giant with the storm strike will remain unchanged

the mod is available at https://yadi.sk/d/gYDyQHKQ35jGNy

Thank you for the mod. As for the giant, yes, it's a bit weak, but then we'd have to swap them. The first upgrade is kind of dark, and the second is fiery, if you look at it from the perspective of the troglodytes, grenadiers, priests, and faceless ones.

I think that in some cases, changes are not necessary. The first ones are traditional iron giants that attack with fire, while the others are giants that use a completely different kind of power – neither darkness nor fire is present in them, and they should be used by "liberal" armies.

Probably, these are creatures from NCF. Here are some mentions (although it's not certain that this is exactly what it is):
/topic/at/842481/
/topic/at/773341/

haven't you heard anything about this mod? I've been looking for it for 5 days, but without success.