SPOILERS!
TURN BACK NOW OR REGRET IT FOR ETERNITY!
Why do you think Zahak and Alenia betray the Prince? From Alenia's dialogue in the beginning, we already know she's a big fan of Godwin politics; Godwin supporters believe punishing Lordlake with the Sun Rune was not enough; they should've been punished further. Punish insurrectionists to the utmost degree!
Zahak is quiet about these political matters, so we don't really know what he's thinking.
Were they already spies for Godwin from the beginning? Or did they suddenly decide to switch sides when the opportunity presented itself?
By the end of the game, Alenia still holds the same beliefs, explaining why she doesn't rejoin the Prince. But how about Zahak? We never really know what's going on in his head.