you have mastered the use of defensive magic, and thus can alter certain spells within the abjuration school of magic in little ways, but the success of the alterations can vary. 

• when you cast the Shield spell you can roll 1d8 instead of using the defined +5 AC bonus
• when you cast the Mage Armor spell you can roll 1d4 to add to the 13 base ac instead of your dexterity modifier