Well, I think all magic users will have an "essence" stat which allows for the amount of mana it has. The Channeler/Sovereign will be the only one which can distill node essence into his own, and increase essence upon level up, but he can spend essence in order to attach it to certain heroes.
In your mentioning of a Mind-Flayer, it could use essence and turn it into an intelligence stat (for mods) or simply have the mind-flayer decrease the mana of a character, and once mana hits 0 (or for most units which have no mana) that creature will die. Sounds like a very scary creature, and another reason for Rare Magical creatures to have large banks of essence/mana.
as far as extra-planar temptations/demons, as usual most units would be completely vulnerable (although I would like for levelled and high-morale units to be quite resistant) ... you could display immunity, Resistance, and Bonus relations via Religion tags. Where Religion A is immune the abilities of Religion B, while B has bonuses vs Religion C, and Religion C has bonuses vs everyone. And Religion D is relatively weak against other religions but is good at breaking morale and insta-killing normal units most effectively. Meanwhile, Relgion C's main rival is Religion A, and Religion C wants to wipe out Religion A. Meanwhile Religion B is the only one in a position to take out Religion C, although would like to wait until C has completely wiped out B, although any increase in mundane power for religion C is bad for religion B, even though B still has the Divine advantage.
Meanwhile, Religion D would want all the other religions to wipe each other out in their own holy wars, as all divine units of religion D could be easily killed by the other religions, although Religion D excells against most other units, so its best used against Massive regular armies ... they want an absence of religion in order for their religion to shine.
There are all sorts of possibilities for the theme of each religion, but the anti-parallel game-mechanic is very dynamic and interesting.
If I was going to assign any "themes" to these religions, I would have Religion C being the all-powerful religion of the Fire-god, while Religion B was the Water-tribe, the only one capable of freezing the flames of the Pyromancers. Meanwhile, Air is practically immune to the effects of water, so could steamroll the water-tribe, but could obviously be burnt by Fire if they went through with their extermination of Water. So Fire would want Air to wipe out Water, and Water would want Fire to wipe out air, meanwhile Earth/Nature would want all these destructive forces to wipe each other out, being Camp D, weak to the other elements, and if the other religions largely wipe each other out, Earth religion will be in a good position to take over the world, or at least convert the rest of the civilized peoples in order to win a religious victory, as its the rationale decision to make in the face of other-wise destruction from the now superior religion.
Equally so, if Religion C went ahead and wiped out Air, Religion B would want to ask Religion D's aid (in the form of regular troops) vs Religion C, for even though Water will beat Fire, Fire now has too much land and troops to be opposed in locations where there are no priests fielded. However, if B and D were to then Wipe out C ... then B would most likely have the advantage. I mean ... whomever has the biggest armies will ultimately be the most powerful, but as far as magical/Divine abilities, its better to have the potential for an absolute advantage than to rather simply happen to have the largest army. AKA people will always want to have a natural advantage, therefore there is more conflict between religions rather than simply to gain the most land ... its also important which religion is going to benefit the most from this war, and an interesting decision of which side to be on ... and what will most likely be the next war once the current enemy is defeated.
The Religion mechanic, combined with the Call-to-Arms mechanic listed else-where, would provide a very interesting Diplomatic arena for the game ... not just as AI modifiers, but also providing a very real dynamic which can be used with Human players on a sophisticated level.