basically the enslave skill lets you turn a normal mob into a mercenary. They will follow you around and you can order them to get, rescue, remove, drop, emote, say. In combat they fight like they would if they were just normally there.
So orc mobs do orc things, warrior mobs do warrior things, etc.
Krunk has the slaver edge which expands the types of races you can enslave. this appears to include evil giants. Personally I would love to run around in the upper mid ranks with Rogash the rock troll, he seems to prog orc things a TON and whip out great damage for his level.