The bartender is found towards the goblin king area, in (surprise) the Bar area. He is surrounded by two Inferno or Fire goblins.
Be very careful in this room, as one wandered can often pull both nearby rooms as well.
For some reason, if you're behind the bar, the creatures infront of the bar will not attack you. (probably very similar to real life ;)
He is sought after by monks, and enchanters who like the legs.