Re-reading the 'Pause' topic gave me an idea, that's been in a few RTS's in differing forms.
The spell would be a slow of sorts, and have an area affect. The choices as to it's ability would be:
Slow units or freeze them?
Can these units be attacked while slowed/frozen?
Single unit targeting or area of attack?
This would give spell casters the chance to get away if they are entrenched in melee combat. It would also give the possibility to slow down retreating units and cut them down with your cavalry.