This is actually a suggestion for multiple spells rather than a single one. Currently, we have a couple combat spells targeted at interrupting a spell in progress (counterspell, spell leach), but most of the tactical spells used by the AI don't have a casting time, so the interrupts aren't very useful.

I suggest 6 new spell ward spells, one for each school of magic cast to ward against tactical spells of the opposite school. So, life mages could cast death ward, water mages could cast fire ward, etc.

The effect of a spell ward would be to double mana cost and increase casting time by 1 of any spell from that school.

So, for instance (and I'm doing these numbers from memory, so forgive me if they're off), a fire mage casting a water ward would cause slow to go from a 5 mana instant cast spell to a 10 mana spell with a casting time of 1. If your enemy began to cast slow, if you have a mage with a sufficient initiative, you would be able to counterspell or spell leach it.

Mages could resist the effects of the ward with a check of their spell mastery versus the spell mastery of the ward caster.