The Storm of Insurrection
Casting Defensively now occurs whenever the caster would like to cast a spell and gain a +4 AC bonus until the beginning of his next turn.
The DC to cast defensively is Concentration DC 15 + 2 x Spell Level
When you fail to cast defensively, you do not automatically lose the spell. Instead you take 2d12 + Spell Level in damage. You must make an additional Concentration check or lose the spell due to damage (DC 10 + damage dealt + spell level).