If the fief is in rebellion and you are in the fief with troops, you use this option to attempt to quell that rebellion. In effect, a "Quell" is the same as a "Pillage" and this is how you do it. You need an army to do quell, of at least 100 troops. The minimum ratio is one soldier for every eighty people in the fief. The more troops you have the easier it is to quell the fief. If you don't have enough troops, the locals will throw you out to an adjacent fief and you'll have to march back in and keep trying until you succeed. A success is equivalent to a pillage. That is, you make some money, but do some damage to the fief in the process.