For people who don't remember exactly, here is the Power Plant location: Abandoned Power Plant
Here is almost exactly where he spawns if you're looking at the power plant map: Inside Power Plant
Here is what you'll see most of the time because his spawn timer is ridiculous
And this is what the beastie looks like: Rakghoul Overlord
As stated largely and in bold, his spawn timer is crazy. We probably waited 2-3 hours last night and nada. I just logged in today to check. Had to switch instances to find him.