Question about Relius in "King Of Attolia"

[sunshinebutter.livejournal.com]

Hi everyone, 

Though this may be have an obvious answer - I have read and re-read the series quite a number of times and love it to bits but the betrayal of Relius the Master of Spies I never quite understood and how Eugenides knew that he was lying when he accused him. 

I believe that I may be reading over it too quickly and not understanding the subtleties that are important to understanding it so I appreciate any feedback on this question; its been plaguing me for a while now. 

I'm expecting almost everyone to have an answer to this. 

Thanks. 

sunshinebutter