My understanding is that the final encounter goes, in order of difficulty, Slave Pens -> Temple -> Arena.
I was with a group that did both the Temple and Arena, but the reward was incredibly lackluster outside of the per member gold, which wasn't significantly different than most "endgame" quests I've done recently. Several of the chests had 9-14 gold pieces in them. While I don't mind the gold, having it be such a pittance versus, say, getting two haste potions from a chest is incredibly discouraging.