An unstoppable spell cannot be countered and is less hindered by spell resistance and effects which might cause you to lose your concentration.

Benefit: An unstoppable spell cannot be countered by another spellcaster even if they successfully identify the spell and cast an appropriate counterspell. A caster who attempts to counter this spell by casting another spell simply loses that spell. In addition, with an unstoppable spell you gain a +2 bonus to level checks made to beat your targets' spell resistance, as well as all concentration checks made to keep from losing the spell. An unstoppable spell uses up a spell slot 1 level higher than the spell's actual level.