Taking some inspiration from classic turn based JRPGs here, Mine Spells act like a counterattack mechanic. When placed down, any incoming damage triggers the spell's effect for the duration of the turn. At the end of the round, the mine is disarmed and must be recast. I think this kind of spell would be a nice addition to the game, as it adds a new layer of mechanics for players to play on, and possibly more cheat options for the developers.
If I were to handle them, they'd probably go something like this: All mine spells cost 4 pips, single target ally. 650 damage, -10% outgoing pierce (on enemy), -35% accuracy 550 damage, -10% outgoing pierce, stun for 1 round 750 damage, -10% outgoing pierce, steal 2 pips 700 damage, -10% outgoing pierce, remove 2-3 wards 600 damage, -10% outgoing pierce, -25% outgoing healing (on enemy) 650 damage, -10% outgoing pierce, +20% universal trap (on enemy) 650 damage, -10% outgoing pierce, -20% outgoing damage (on enemy)
Astral and shadow versions for monsters, perhaps. 550 damage, -10% outgoing pierce, fortify 600 damage, -10% outgoing pierce, Hidden charm, trap, or ward? 400 damage, -10% outgoing pierce, Shift Shadow? (350 damage, +30 to backlash, if any) 650 damage, -10% outgoing pierce, +10% outgoing pierce (on self)
I'd also probably put these on a secret trainer somewhere, or maybe a side quest.
We do have something sort of like this with that spell that gives you pips when you get hit above a certain damage threshold, though I agree that we should expand upon the concept. These all look good to me, I would love to get a free trap just for getting hit lol