Cursed Blade(s); Summons a Balance Blade that, on the next turn, becomes a Weaken Blade, then again, shifts back to a Balance Blade, and so on until it's used. This blade can be cast on anyone in the battle, both sides.
This can be a fantastic combo weapon for supports who want to give a teammate another boost with all sorts of others, or instead be cast on an enemy for weakening them in the next turn (if your turn went last.) Strategies involving such a spell can even work in solo play, where a player can cast the Cursed Blade on themselves, and attack on the turn it's actively a Balance Blade.
This could be very interesting to see in PvP, giving a rapidly shifting threat to both sides on either end, a wizard could still just fire through, it won't stop them but weaken them, or if their spell fizzles, they can use it with a boost on their next turn. A gamble for the caster, safety net and a curse.