I just wanted to write some spell ideas.
Scatter (Dimensional II, Sorcery)
Each unit in the target tile has a 10% chance to be moved to a random adjacent tile.
Sever (Dimensional III, Sorcery)
All summoned units in the target plot are destroyed. All units in it gain a 'casted' promotion that lingers until the end of their owners' turn, preventing spellcasting.
Phantasm (Mind II, Summoning)
Summons one immobile and magic immune 8-strength unit with fear, but it dies as soon as it takes any damage whatsoever.
Emphatic Manifestation (Mind III, Summoning)
Summons a 14-strength unit with movement 1 and hidden nationality. It is permanent, but it takes some damage at the end of each turn away from its caster. If either the caster or the manifestation should die, so does the other. When in the same tile, the caster can reabsorb the manifestation, gaining 25% of any experience it had aquired.
Drown (Water II, Divine)
Converts one target living non-animal melee unit that is near water into a full-health The Drown unit. Targets owned by the caster are always affected, enemies get a chance to resist it.
(Something like: 80%+2%/level near rivers, 60%+2%/level near lakes and coasts, 40%+2%/level on lakes, oasises and coasts, 10%+2%/level in oceans. Double all numbers for heroes.)
Alternative (rare) Domination failure effect: Exchange control of the involved units. The target gains all experience, levels and promotions of the caster, and the caster gains all experience, levels and promotions of the target.
("Okay, so I failed to steal that berserker outright... but now, my Archmage has 13 strength and collateral damage!")
Bless: Weakened and shuffled over to Law II.
Last Stand (Spirit III, Divine)
The caster is restored to full health while losing all remaining moves.
If the caster's stack is attacked, he is always the chosen defender.
If the caster wins a combat, he is restored to full health.
If the caster loses a combat, he is restored to full health while the winner withdraws.
At the beginning of your next turn, if the caster lost a combat, he drops dead.
If he did not, he instead loses one point of strength permanently.
Special order-specific spell only castable with several elements learned:
Blaze of Glory (Divine, Fire III, Law III, Spirit III)
All damage dealt to any unit you control is instead dealt to the caster of this spell. Once this has been cast, no healing spells will work and any immortal promotions or lifesparks are ignored when death does arrive.