The Overking is in the basement along with the Queen and Prince. It's not all that hard to describe, but actually getting to them takes some work, as there are 2 locked doors before them that take a rogue to open.
Enter Chardok and head deep inside, when you get to the suspended bridge area with the Golems, head right and you will eventually come to an area where you can't go any further with a locked door, That's the first entrance to the Overking area.
The easiest way to do this any more is have someone with good faction pull the overking up to the zone out, or at least away from all the mobs there.