Master of War is a proposed new office in the Small Council of the Seven Kingdoms which Cersei Lannister made up despite lacking the authority to do so. Cersei invited her uncle Kevan Lannister to serve on King Tommen's new Small Council, and given his position as one of the main commanders of the Lannister armies, she offered him a position as "Master of War" - though there was no such thing as a "Master of War" previously, and the realm's armies were commanded by the Protector of the Realm, usually the king himself. Kevan realized Cersei was making a grab for power and stacking the Small Council with sycophants, so he declined the offer and left.