In a game, things whose utility varies with the kinds of opponents you face have serious issues.Variants that target certain types of spells.
Players rarely have control over it. So you either need to be able to scout & prepare, or just get lucky.
So this kind of spell ends up taking a slot for a non-prepared caster that can only be changed like once per level. For a prepared caster it is slightly more plausible, but still needs a bit of "scry to win".
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Ideas:
Mage Duel
- Starts a "duel" between you and the target; no save.
- Requires concentration.
- If you have concentration, you have to roll at the start of your turn to keep it.
- You can both burn spell slots to disrupt enemy spells. If you have a spell of the same school you get a bonus; if it also matches the level you get another bonus.
- Instead of yes/no, how about 4 different results:
** No
** Yes
** Caster takes damage, spell goes off
Spell Decay
* Increases the rate at which time passes before the duration of a spell ends