The Slavemastery skill is a class skill that the Warlord class gets. It is used to enslave (tame) Humanoid monsters such as Humans and Elves.
As with the other taming skills you can also use bonuses on items to acquire this skill although it is unlikely you will be able to tame very high level monsters doing it that way.
Skill Use
To enslave a humanoid you must first find one, if you have the Track Humanoids skill you can use that to find one or you can simply rely on coming across one by chance.
When you move the mouse over a monster it will tell you its classification in the middle of the screen so it is fairly simple to spot humanoids it will also list how much skill and what skill is needed to enslave this particular monster.
If you have enough skill simply click the button marked Enslave in the middle bank of buttons after selecting the monster. This will tame the pet and make it available to you on the Minions > Manage screen, you will gain a purple skill gain and some experience for taming.
By default you have a total of 4 pet slots, this is broken down as 1 pet in stables and 3 pets out of stables. You can purchase additional pet slots via the File > Buy Extras > Enhancements screen.
Formula
Skill required = (Monster Level * 5)
i.e. To tame a level 100 monster you need 500 skill.