You are able to trick your opponents into believing you are casting a spell in order to surprise them with an attack instead.

Prerequisite(s): Int 13, Bluff 5 ranks, and Spellcraft 5 ranks, 3rd-level Arcane Trickster

Benefit: By expending an unused 1st-level or higher spell (or spell per day for spontaneous casters) when you feint an opponent, you gain a bonus to your Bluff check equal to the level of the spell expended. If the feint is successful, your attack deals an additional +1d6 sneak attack damage per three levels of the spell expended (minimum +1d6), and your opponent cannot make attacks of opportunity against you when you cast real spells for the duration of the encounter.