Well, I don't know how usefull it is.. But there's a mob in Netherstorm called Eye of Culuthas (Eye of Culuthas - NPCs - World of Warcraft) which you can enslave as a warlock, it hits for 700-900 and puts a debuff on your target that increases spell damage taken by 50% (big shadowbolt crits)
You can use this for killing other mobs, for an example the eye of Culuthas drops mote of shadow, so should be pretty easy to farm with this mob enslaved.. Although, enslave has diminshing returns on it, so you'll have to kill it at some point and find a new one.
I'll post some screenshots of it enslaved later today with tooltip on what it can do.