A protective spirit is an invisible, mindless, shapeless force that hovers about you, defending you against unexpected attacks. When a creature makes an attack of opportunity against you, the protective spirit makes an immediate attack using your base attack bonus plus your Dexterity modifier. On a successful hit, the spirit does no damage, but it causes the attack of opportunity to automatically miss. A protective spirit can defend against a number of attacks of opportunity equal to your Dexterity bonus (minimum 1) per round.
A protective spirit occupies your space, and moves with you. It cannot be attacked or harmed by physical attacks, but dispel magic, disintegrate, a sphere of annihilation, or a rod of cancellation affects it. A protective spirit's AC against touch attacks is 10 + your Dexterity modifier.
Section 15: Copyright Notice - Advanced Player's Guide