The current Overlord has held the position for many centuries, and He has kept Hell tightly under HIs control. He has no specific sphere of influence, but rather oversees the other Princes and directs when and where, and how much, Evil flows into the Balance. This particular Overlord has never been seen except under a heavy, hooded cloak, with a skeletal pair of hands. He does not speak out loud, but only by telepathy. Former Overlords include Lucifer, Satanas, and Moloch.
And Azbaelus, why, you should be reading Trials of a Demon Prince, and you’ll get that story soon enough.