I was figuring one of two things

i) comedically infusing a gem with some looping message to remind them what the fuck they're meant to do that he sticks on them like a badge of shame until they're done or
ii) He could use the mysterious ability of 'channelling magic' that Warcraft NPCs use on anything and everything to sometimes shock them into staying on task

But tbh, if the necromancers can be persuaded Wrathion considers himself very persuasive, and that is probably the best way to ensure... the least chaos.