If a unit routs in the final battle for a castle (Storming) it will retreat to a nearby province. If you kill the God and he has no dominion, suddenly those units 'disappear' (Prophet is the main culprit).

So if the caster of the original spell 'routed' instead of died or in some way got away (if it wasn't the God who cast the spell) the effect will be there for the length of the game until they are overpowered or dispelled.