I really wish he was more fleshed out.. he reminded me of brother what's his name in IWD (which I suppose is fitting, since you end up fighting Belhifet again) who was one of my favourite characters due to how sinister he seemed.
What I didn't really like is how little you learn about his motives and how a lot of things you do don't really seem to affect him in-game.
The necromantic circle under the river caves, for example. All signs point to Hephernaan raising an undead army, you foil his plans, but this doesn't really seem to have any consequences. You DO get one dialogue option during (I think) the parley, but it doesn't really affect anything.
Also, the goal of the undead army wasn't explained.. and later you find out he wanted to release an army of demons on the Sword Coast.
I mean, his motives seem very shady and the only vibe I got was the typical "evil bastard" who wanted to sow chaos. Admittedly, he did make an agreement with a demon, so there must have been something behind it, but what?
Now a short disclaimer: I played the last two chapters while ill and tired, so I might have just skimmed a lot of text and things might not have registered with me....
Anyway, fill me in if I'm wrong!
Edit: slightly relevant right?