It seems like your character is pretty suspect and pretty much figures it out when he just appears in the sewers using magic. The voice actors seem pretty suspicious anyway, check it out during the extra dialogue options when you see him down there. In my opinion this is when dat wool be pulled from da eyes.
Oh yeah, the theme is ongoing literally from the moment you step into act 2. One of the very first pieces of dialogue is, "The emperor hasn't been himself lately..." and the response, "Everything is not as it seems."
If you talk to Hakan on some of the quests where he appears to you remotely he informs you that his communication is made possible through some object he stole from one of his captors. My templar I had with me at the time makes a sceptical remark about how many captors he had, how would he even be able to sneak past them to steal the object?
I didn't figure it out until my hero called the kid Belial.. WTF?! Í literally shat my pants at the genius of the storyline. Sometimes being a bit slow pays off.
There are some Lore books in the inn at Caldeum and the servant who fell to his death that shed a little light on Hakan. But as soon as he started appearing in places that a prince shouldn't have been then it became immediately obvious (and as others have said, very cheesy hollywood'esque). Also, the fact that Belial is the the Lord of Lies raised suspicions around Hakan as well.
It was obvious to us playing the game, and we are our character, therefor it is obvious to your character as well... Like, could you imagine how retarded it would've been if there was a "omg no way, HE'S belial?!?!?!?!?" moment for all the characters at the end of the act, we all would've been sitting there saying "yeah no shit", its not a twist, its obvious, anything other than your character being fed up and going "ffs your obviously belial dickhead your not fooling anyone" would've been a slap in the face to your characters intelligence.
Belial himself being a retard and making it so obvious is a questionable part of the story, not how your character managed to figure out it's Belial.
DUN DUN DUNNNNNNN! Adria was actually Belial all along, we killed an innocent kid, left a city to burn without it's leader, and only ended up killing 6 of the 7 Evil Lords. At the end of the game we'll realise that dude with the hairpiece was Bruce Willis the whole time...
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