I'm a bit unclear how this works in the Enhanced Edition...
Early on in the game, in the Hive, you're given Moridor's Box. There are 3 ways to 'resolve' this quest:
1. Open the box. This unleashes a lesser fiend on you.
2. Follow the quest chain. This 'banishes' the fiend.
3. Leave the Hive. This unleashes the fiend.
Much later in the game, in Curst Gone, you can potentially encounter the fiend - it's super powerful but drops an awesome item and gives insanely high experience if you kill it.
However, I read conflicting reports online of what it takes for it to appear. Most sources say it will only appear if you resolve the quest using method 3, but some say 2 or 3 do the same thing.
So what is it? What are the conditions I have to meet to be able to encounter the fiend? Personally I'm hoping it could potentially be method 2 since that nets more experience early in the game and is a more satisfying conclusion to the quest overall, but if I have to do 3 to see the fiend I'll do that.