How about cards that can be cast over a hex, and are hidden from opponents, but when the opponent walks over one, it releases some kind of trap, like

Bear Trap

DMG 1
Creature loses next turns actions
Walking Distance reduced to 1 hex

or

Stinging Nest

DMG 1
Sight Reduced to zero
Any future damage taken increased due to stinging welts

kind of like the tangler i guess

these would be useful cause you could set up a hodgepodge defensive grid.
Also the casting players creatures shouldn't be able to walk all over the traps, they should have to avoid them...

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Those could be fun...

For gameplay terms, I'd imagine they'd have to count as enchantments, but be invisible to the other player and once used the enchantment would have to disappear.

Unless someway of detecting them is added in, I don't think they could be powerful though. Don't want something that could completely decimate an army with no way to stop it.

I agree that they should be more nuisance than deadly, thats what i tried to infer by showing they only did one damage.

You could create creatures with special trap detect abilities (ie dwarves would have this advantage in the mountain traps, sylvan in the forests, etc,)

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you'd need hovering traps as well, like 'rain cloud' or something.

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I think they would probably be enchant worlds that held the information of there location in private, i think this would be a great idea, ive been trying to do just this same thing for a tactics game ive been developing but it doesnt translate as well in a non digital envirement.