[Vicious Love]

Only have 2 metric minutes to post, so I'll be brief.
How about an alteration combat spell, in 3 possible variants(Single target, large area, and Inner Sun style death curse) which can only be cast/triggered underwater, and renders the target/s aquatic? Amphibians, poor amphibians, water mages and their retinues, anyone who fails to resist would be incapable of leaving the ocean without an amulet of the fish. Stylish, appropriate, and only rasies 1 question, that of whether poor amphibians would retain their penalties for fighting underwater.
A similar spell with slightly different style could simply afflict target water mages/amphibians/aquatic units with the mermen's curse, meaning they'd have to return to water every so often, amulets of the fish nonwithstanding. If this spell had a small area effect(Meant to be used on commanders) but could be used on land based units ordinarily incapable of entering the sea, that might serve to complicate things nicely.
I actually got the idea for this curse when toying with a nature battlefield enchantment, "Circe's Caprice". Irreversible, insanely random(And affected by fortune scale) mix of Polymorph, Transformation and Cross Breeding, which affects both sides in any given battle(Including the caster, but that's what mage MR is for. Besides, the effects aren't necessarily negative). If we start grasping at straws, I can see how a, shall we say, fish-based variant of that might work, though I imagine actually coding this sort of thing could be a major headache.
The second spell could also have a blood/nature or blood/earth counterpart which binds units/commanders to a particular province, a la hamadryads, certain vampire myths, and much, much more. Not that that's on topic, or anything, I just find it cool.

Time elapsed! See ya.