A bit of a serious spoiler regarding Xardas and his goals:
I can tell you this much - his plan involving the nameless hero was not to just betray him to get the power of beliar, or to get world dominance.. he still has the main hero in his plans, and unlike anyone else in the realm he actually wants to remove the influence of the gods themselves from this world.. YOU can join him in doing that, and complete the game with Xardas as your ally.
As usual he doesn't do any needless talking, so when you reach him you won't get a long speech with explainations and answers to questions long overdue - instead he tells you right away what needs to be done.
What his plans are and what needs to be done is up to you to find out.