Slaves are purchased with the red blood. They acquire blood (income) for you that you are paid hourly and are essential to the success of your game. Without them, you cannot afford to buy powers and without powers you can't successfully fight.
Every time you purchase a Slave, the cost goes up for that Slave. Some Slaves require "clan minders" to "oversee" them. You begin the game with minders to manage your slaves and that amount stays the same as you progress. You must balance the amount of minders available to the most lucrative slaves for maximum blood income.
Different slaves open as you progress through the levels. Each one that you unlock will produce more blood than the one previous. You can sell your slaves at any time, but be aware that you do not get the full price back for them.
Note: if you sell too many slaves, and your power costs exceed your income, your banked blood will be used to pay for your powers. Once your blood is gone, your powers will start to disappear since you can no longer afford them.