The dragonmark shield hovers in front of you and moves with you. It negates magic missile attacks directed at you. In addition, it provides a shield bonus to AC and a bonus on Reflex saves based on the strength of your dragonmark.
The shield bonus to AC applies against incorporeal touch attacks, since the dragonmark shield is a force effect. The dragonmark shield has no armor check penalty or arcane spell failure chance.
If you use an immediate action to invoke the dragonmark shield, it disappears at the end of your next turn.