The Master's Staff is part of the Minions Mod. It is used to summon and control Minions.
To gain the Master's Staff, you must commit evil 4 times. Press G, and press the button saying 'Commit to evil'. Select the option that you think is the most evil, and each time a malecious voice will tell you that 'The Gods are pleased with your sacrifice.' After completing this 4 times, you will be given a Master's Staff.